emacs: Add 'guix-find-file'.

* emacs/guix-utils.el (guix-find-file): New procedure.
* emacs/guix-info.el (guix-file): Use it.

emacs: Add interface for comparing generations.

Suggested by Ludovic Courtès.

* doc/emacs.texi (Emacs List buffer): Document new key bindings.
* emacs/guix-base.el (guix-generation-packages-buffer-name-function,
  guix-generation-packages-update-buffer, guix-output-name-width): New
  variables.
  (guix-generation-file, guix-manifest-file, 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-package, guix-generation-insert-packages,
  guix-profile-generation-manifest-file,
  guix-profile-generation-packages-buffer): New procedures.
* emacs/guix-list.el: Add key bindings for comparing generations.
  (guix-generation-list-generations-to-compare,
  guix-generation-list-show-added-packages,
  guix-generation-list-show-removed-packages, guix-generation-list-compare,
  guix-generation-list-ediff-manifests, guix-generation-list-diff-manifests,
  guix-generation-list-ediff-packages, guix-generation-list-diff-packages,
  guix-generation-list-ediff, guix-generation-list-diff): New procedures.
* emacs/guix-messages.el (guix-messages): Add 'generation-diff' search type.
  (guix-message-outputs-by-diff): New procedure.
* emacs/guix-utils.el (guix-diff-switches): New variable.
  (guix-diff): New procedure.
* emacs/guix-main.scm (package/output-sexps): Handle 'generation-diff' search
  type.
  (manifest-entry->package-specification,
  manifest-entries->package-specifications, generation-package-specifications,
  generation-package-specifications+paths, generation-difference): New
  procedures.

emacs: Add 'guix-generations-by-time' command.

* emacs/guix-main.scm (find-generations): Add finding generations by time.
* emacs/guix-utils.el (guix-read-date): New procedure.
* emacs/guix-base.el (guix-messages): Add new messages.
* emacs/guix.el (guix-generations-by-time): New command.
* doc/emacs.texi (emacs Commands): Document it.

emacs: Support font-locking.

Avoid breaking highlighting after adding new font-lock keywords.

* emacs/guix-base.el (guix-insert-package-strings): Use 'propertize' instead
  of 'guix-get-string'.
* emacs/guix-info.el (guix, guix-action, guix-file, guix-url,
  guix-package-location, guix-package-name): New button types.
  (guix-info-insert-action-button, guix-info-insert-file-path,
  guix-info-insert-url, guix-package-info-insert-location,
  guix-package-info-insert-full-names,
  guix-package-info-insert-non-unique-text): Adjust for 'guix-insert-button'
  and button types.
  (guix-package-info-name-button): New face.
  (guix-package-info-define-insert-inputs): Use it.  Add new button types.
  (guix-package-info-insert-full-name): Remove.
* emacs/guix-utils.el (guix-get-string): Replace 'face' with 'font-lock-face'.
  (guix-insert-button): Adjust for using button types.

Merge branch 'master' into core-updates

Add Emacs user interface.

* configure.ac (emacsuidir): New variable.
  (AC_CONFIG_FILES): Add 'emacs/guix-init.el', 'emacs/guix-helper.scm'.
* Makefile.am: Include 'emacs.am'.
* emacs.am: New file.
* doc/emacs.texi: New file.
* doc/guix.texi: Include 'emacs.texi'.
* emacs/guix-backend.el: New file.
* emacs/guix-base.el: New file.
* emacs/guix-helper.scm.in: New file.
* emacs/guix-history.el: New file.
* emacs/guix-info.el: New file.
* emacs/guix-init.el.in: New file.
* emacs/guix-list.el: New file.
* emacs/guix-main.scm: New file.
* emacs/guix-utils.el: New file.
* emacs/guix.el: New file.