emacs: Complete --file in 'guix build' popup.

* emacs/guix-command.el (guix-command-improve-build-argument): Use
  'guix-read-file-name' to complete --file option.

emacs: Add missing shell completions.

* emacs/guix-pcomplete.el (guix-pcomplete-complete-option-arg): Complete
  file name for '--install-from-file' option of 'guix package' command,
  and for '--file' and '--with-source' options of 'guix build' command.

emacs: Display supported systems in "Package Info".

* emacs/guix-main.scm (%package-param-alist): Add 'systems'.
* emacs/guix-ui-package.el (guix-package-info-insert-systems): New
  procedure.
  (guix-package-info-format): Add it.
  (guix-output-info-format): Likewise.
  (guix-package-info-titles): Add "Supported systems" title.

emacs: Use "B" key in "Package List" to display latest builds.

* emacs/guix-ui-package.el (guix-package-list-latest-builds): New
  procedure.
  (guix-package-list-mode-map): Bind "B" key to it.
  (guix-output-list-mode-map): Likewise.
* doc/emacs.texi (Emacs List buffer): Document it.

emacs: Add Hydra interface.

* emacs/guix-utils.el (guix-hexify, guix-number->bool): New procedures.
  (guix-while-null): New macro.
* emacs/guix-hydra.el: New file.
* emacs/guix-hydra-build.el: New file.
* emacs/guix-hydra-jobset.el: New file.
* emacs.am (ELFILES): Add them.
* doc/emacs.texi (Emacs Hydra): New node.
  (Emacs Interface): Add it.
* doc/guix.texi (Top): Add it.
  (Substitutes): Mention Emacs interface.

emacs: Add 'guix-ui-read-profile'.

* emacs/guix-ui.el (guix-ui-read-profile): New procedure.
* emacs/guix-ui-package.el (guix-search-by-name)
  (guix-search-by-regexp, guix-installed-packages)
  (guix-obsolete-packages, guix-all-available-packages)
  (guix-newest-available-packages): Use it.
* emacs/guix-ui-generation.el (guix-generations)
  (guix-last-generations, guix-generations-by-time): Use it.

emacs: Add hierarchy of customization groups.

* emacs/guix-buffer.el (guix-define-groups, guix-define-entry-type)
  (guix-define-buffer-type): New macros.
  (guix-buffer-define-interface): Add parent groups for the generated
  custom groups.
* emacs/guix-info.el: Use 'guix-define-buffer-type' to generate custom
  groups.
* emacs/guix-list.el: Likewise.
* emacs/guix-ui.el: Use 'guix-define-groups' to generate custom groups.
  (guix-ui-define-entry-type): New macro.
* emacs/guix-ui-package.el: Use it.
* emacs/guix-ui-generation.el: Use it.

emacs: Return nil instead of "" in completing readers.

* emacs/guix-utils.el (guix-completing-read): New procedure.
  (guix-define-readers): Use it.

emacs: Add 'guix-list-get-display-entries'.

* emacs/guix-list.el (guix-list-get-display-entries): New procedure.
* emacs/guix-ui-generation.el (guix-generation-get-display): Use it.

emacs: Add 'list-single' variables.

* emacs/guix-list.el (guix-list-single-entry?): New procedure.
  (guix-list-define-interface): Add ':list-single?' key and generate
  'guix-ENTRY-TYPE-list-single' variables.
* emacs/guix-ui-package.el (guix-package-list-single): Remove.

emacs: Reorganize package/generation UI code.

Move the code for packages/generations interface from "guix-info.el",
"guix-list.el", "guix-base.el" and "guix.el" to "guix-ui-package.el" and
"guix-ui-generation.el".

* emacs/guix-base.el (guix-package-entry->name-specification)
  (guix-package-entries->name-specifications)
  (guix-package-id-and-output-by-output-id)
  (guix-package-installed-outputs, guix-process-package-actions)
  (guix-package-list-type, guix-package-info-type)
  (guix-continue-package-operation-p, guix-get-package-strings)
  (guix-insert-package-strings): Move to "guix-ui-package.el".
  (guix-generation-packages-buffer-name-function, guix-output-name-width)
  (guix-generation-packages-update-buffer, guix-generation-packages)
  (guix-generation-packages-buffer-name-default)
  (guix-generation-packages-buffer-name-long)
  (guix-generation-packages-buffer-name, guix-generation-packages-buffer)
  (guix-generation-insert-packages, guix-generation-insert-package)
  (guix-profile-generation-manifest-file, guix-delete-generations)
  (guix-profile-generation-packages-buffer, guix-switch-to-generation):
  Move to "guix-ui-generation.el".
* emacs/guix-info.el (guix-package-location, guix-package-name)
  (guix-package-source, guix-package-info-source)
  (guix-package-info-heading, guix-package-info-license)
  (guix-package-info-name, guix-package-info-name-button)
  (guix-package-info-version, guix-package-info-location)
  (guix-package-info-synopsis, guix-package-info-description)
  (guix-package-info-obsolete, guix-package-info-installed-outputs)
  (guix-package-info-uninstalled-outputs)
  (guix-package-info-insert-heading)
  (guix-package-info-define-insert-inputs)
  (guix-package-info-obsolete-string)
  (guix-package-info-insert-obsolete-text)
  (guix-package-info-insert-non-unique-text)
  (guix-package-info-insert-outputs, guix-package-info-insert-output)
  (guix-package-info-insert-action-button)
  (guix-package-info-auto-find-source)
  (guix-package-info-auto-download-source)
  (guix-package-info-download-buffer, guix-package-info-show-source)
  (guix-package-info-download-source, guix-package-info-insert-source)
  (guix-package-info-redisplay-after-download)
  (guix-output-info-insert-version, guix-output-info-insert-output):
  Move to "guix-ui-package.el".
  (guix-generation-info-number, guix-generation-info-current)
  (guix-generation-not-current, guix-generation-info-insert-number)
  (guix-generation-info-insert-current): Move to "guix-ui-generation.el".
* emacs/guix-list.el (guix-package-list-generation-marking-enabled)
  (guix-package-list-installed, guix-package-list-obsolete)
  (guix-package-list-get-name, guix-package-list-get-installed-outputs)
  (guix-package-list-marking-check, guix-package-list-mark-outputs)
  (guix-package-list-mark-install, guix-package-list-mark-delete)
  (guix-package-list-mark-upgrade, guix-package-list-mark-upgrades)
  (guix-list-mark-package-upgrades, guix-list-execute-package-actions)
  (guix-package-list-execute, guix-package-list-make-action)
  (guix-package-list-edit, guix-output-list-mark-install)
  (guix-output-list-mark-delete, guix-output-list-mark-upgrade)
  (guix-output-list-mark-upgrades, guix-output-list-make-action)
  (guix-output-list-describe, guix-output-list-edit): Move to
  "guix-ui-package.el".
  (guix-generation-list-get-current, guix-generation-list-switch)
  (guix-generation-list-generations-to-compare)
  (guix-generation-list-compare, guix-generation-list-show-packages)
  (guix-generation-list-show-added-packages)
  (guix-generation-list-show-removed-packages)
  (guix-generation-list-diff, guix-generation-list-diff-manifests)
  (guix-generation-list-ediff, guix-generation-list-ediff-manifests)
  (guix-generation-list-diff-packages)
  (guix-generation-list-ediff-packages)
  (guix-generation-list-mark-delete, guix-generation-list-execute): Move
  to "guix-ui-generation.el".
* emacs/guix.el: Remove.
  (guix, guix-faces, guix-edit): Move to "guix-base.el".
  (guix-list-single-package, guix-search-params, guix-search-history)
  (guix-get-show-packages, guix-search-by-name, guix-search-by-regexp)
  (guix-installed-packages, guix-obsolete-packages)
  (guix-all-available-packages, guix-newest-available-packages): Move
  to "guix-ui-package.el".
  (guix-get-show-generations, guix-generations, guix-last-generations)
  (guix-generations-by-time): Move to "guix-ui-generation.el".
* emacs.am (ELFILES): Remove "guix.el".  Add "guix-ui-package.el" and
  "guix-ui-generation.el".
* doc/emacs.texi (Emacs Appearance): Adjust accordingly.

emacs: Reorganize 'readers' code.

* emacs/guix-base.el (guix-graph-type-names, guix-refresh-updater-names)
  (guix-lint-checker-names, guix-package-names): Move to...
* emacs/guix-read.el: ... here.
  (guix-read-file-name, guix-define-reader, guix-define-readers): Move to...
* emacs/guix-utils.el: ... here.

emacs: Generalize buffer interface.

Extract the code for defining buffer interface from "guix-base.el",
generalize it and move to "guix-buffer.el".

* emacs.am (ELFILES): Add "emacs/guix-buffer.el".
* emacs/guix-base.el (guix-profile, guix-entries, guix-buffer-type)
  (guix-entry-type, guix-search-type, guix-search-vals, guix-set-vars)
  (guix-get-symbol, guix-show-entries, guix-get-show-entries)
  (guix-set-buffer, guix-history-call, guix-make-history-item)
  (guix-get-params-for-receiving): Remove.
  (guix-switch-to-buffer): Rename to 'guix-buffer-display' and move to
  "guix-buffer.el".
  (guix-get-entries): Rename to 'guix-ui-get-entries' and move to
  "guix-ui.el".
  (guix-buffer-data, guix-buffer-value, guix-buffer-param-title)
  (guix-buffer-name, guix-buffer-history-size)
  (guix-buffer-revert-confirm?, guix-buffer-map, guix-buffer-revert)
  (guix-buffer-after-redisplay-hook, guix-buffer-redisplay)
  (guix-buffer-redisplay-goto-button): Move to...
* emacs/guix-buffer.el: ... here.  New file.
  (guix-buffer-item): New variable.
  (guix-buffer-with-item, guix-buffer-with-current-item)
  (guix-buffer-define-current-item-accessor)
  (guix-buffer-define-current-item-accessors)
  (guix-buffer-define-current-args-accessor)
  (guix-buffer-define-current-args-accessors): New macros.
  (guix-buffer-get-entries, guix-buffer-mode-enable)
  (guix-buffer-mode-initialize, guix-buffer-insert-entries)
  (guix-buffer-show-entries-default, guix-buffer-show-entries)
  (guix-buffer-message, guix-buffer-history-item, guix-buffer-set)
  (guix-buffer-display-entries-current)
  (guix-buffer-get-display-entries-current)
  (guix-buffer-display-entries, guix-buffer-get-display-entries): New
  procedures.
* emacs/guix-info.el: Adjust for the procedures renaming.
  (guix-info-define-interface): Add ':show-entries-function' keyword.
* emacs/guix-list.el: Likewise.
* emacs/guix-ui.el (guix-ui-define-interface): Generate
  'guix-ENTRY-TYPE-BUFFER-TYPE-get-entries' procedure based on
  'guix-ui-get-entries'.
* emacs/guix.el (guix-get-show-packages, guix-get-show-generations):
  Adjust for the procedures renaming.

emacs: info: Get rid of syntactic fontification.

* emacs/guix-info.el (guix-info-mode-initialize): New procedure.  Set
  'font-lock-defaults' to avoid syntactic fontification.
  (guix-info-define-interface): Use it.

emacs: Generalize buffer naming.

* emacs/guix-base.el (guix-buffer-name): New procedure.
  (guix-buffer-define-interface): Make ':buffer-name' a required keyword.
  (guix-update-after-operation, guix-buffer-name-function)
  (guix-buffer-name-simple, guix-buffer-name-default, guix-buffer-name)
  (guix-buffer-p, guix-buffers, guix-update-buffer)
  (guix-update-buffers-maybe-after-operation): Adjust, move and rename to...
* emacs/guix-ui.el (guix-ui-update-after-operation)
  (guix-ui-buffer-name-function, guix-ui-buffer-name-simple)
  (guix-ui-buffer-name-default, guix-ui-buffer-name)
  (guix-ui-buffer?, guix-ui-buffers, guix-ui-update-buffer)
  (guix-ui-update-buffers-after-operation): ... this.
  (guix-ui-define-interface): Generate
  'guix-ENTRY-TYPE-BUFFER-TYPE-buffer-name' procedure and pass it as
  ':buffer-name' argument.
  (guix-ui): New custom group.
* emacs/guix-info.el: Specify ':buffer-name' for the defined interfaces.
* emacs/guix-list.el: Likewise.
* doc/emacs.texi (Emacs Appearance): Adjust accordingly.

emacs: info: Buttonize package name heading.

* emacs/guix-info.el (guix-package-info-insert-heading): Insert
  buttonized name specification.

emacs: Improve messages for packages found by ID.

* emacs/guix-messages.el (guix-message-packages-by-id): New procedure.
  (guix-messages): Use it.

emacs: Generalize buffer reverting.

This is a partial revert of commit d2b299a91f3be283df1264afe62770ab2783ace9.

* emacs/guix-base.el (guix-revert-buffer): Make it more general by
  removing the code specific to a particular search (a search for
  packages/outputs by ID).  Rename to...
  (guix-buffer-revert): ... this.

emacs: Generalize buffer redisplaying.

* emacs/guix-base.el (guix-buffer-after-redisplay-hook): New variable.
  (guix-redisplay-buffer): Use it.  Remove all arguments and the code
  for moving point to the next button.  Rename to...
  (guix-buffer-redisplay): ... this.
  (guix-buffer-redisplay-goto-button): New procedure.
* emacs/guix-info.el (guix-package-info-show-source): Use it.  Adjust
  accordingly.
  (guix-package-info-redisplay-after-download): Likewise.

emacs: Split 'guix-root-map' keymap.

* emacs/guix-base.el: (guix-root-map): Split into...
  (guix-buffer-map): ... this and...
* emacs/guix-ui.el (guix-ui-map): ... this.
  (guix-ui-define-interface): Generate
  'guix-ENTRY-TYPE-BUFFER-TYPE-mode-map' keymaps inheriting them from
  'guix-ui-map' and 'guix-BUFFER-TYPE-mode-map'.
* emacs/guix-info.el (guix-info-mode-map): Use 'guix-buffer-map'.
* emacs/guix-list.el (guix-list-mode-map): Likewise.
* doc/emacs.texi (Emacs Keymaps): Document new keymaps.