monads: Allow resolution of a monad's bind/return at expansion time.

* guix/monads.scm (<monad>): Turn in a raw SRFI-9 record type.
  (define-monad): New macro.
  (with-monad): Add a case for when MONAD is a macro.
  (identity-return, identity-bind, store-return, store-bind): Inline.
  (%identity-monad, %store-monad): Use 'define-monad'.
* tests/monads.scm ("monad?"): New test.

gnu: vm: Rewrite helper functions as monadic functions.

* gnu/system/dmd.scm (host-name-service, nscd-service, mingetty-service,
  syslog-service, guix-service, static-networking-service): Rewrite as
  monadic functions.
  (dmd-configuration-file): Use 'text-file' instead of
  'add-text-to-store'.
* gnu/system/grub.scm (grub-configuration-file): Rewrite as a monadic
  function.
* gnu/system/linux.scm (pam-services->directory): Likewise.
* gnu/system/shadow.scm (group-file, passwd-file, guix-build-accounts):
  Likewise.
* gnu/system/vm.scm (expression->derivation-in-linux-vm, qemu-image,
  union, system-qemu-image): Likewise.

Add (guix monads).

* guix/monads.scm: New file.
* tests/monads.scm: New file.
* Makefile.am (MODULES): Add guix/monads.scm.
  (SCM_TESTS): Add tests/monads.scm.
* doc/guix.texi (The Store Monad): New node.
  (The Store): Reference it.

gnu: libxcb: Enable xcb-xkb.

* gnu/packages/xorg.scm (libxcb): Add configure flag "--enable-xkb".

gnu: gnome: Add brasero.

* gnu/packages/gnome.scm (brasero): New variable; not totally usable due to
    lacking inputs.

gnu: Add hicolor-icon-theme.

* gnu/packages/gnome.scm (hicolor-icon-theme): New variable.

gnu: gnome: Add gnome-icon-theme.

* gnu/packages/gnome.scm (gnome-icon-theme): New variable.

gnu: Add icon-naming-utils.

* gnu/packages/gnome.scm (icon-naming-utils): New variable.

gnu: Add perl-xml-simple.

* gnu/packages/xml.scm (perl-xml-simple): New variable.

gnu: gnome: Add gsettings-desktop-schemas.

* gnu/packages/gnome.scm (gsettings-desktop-schemas): New variable.

gnu: xnee: Drop input gtk+.

* gnu/packages/xnee.scm (xnee): Drop unused input gtk+.

gnu: itstool: Really push the modification announced in 9000d556.

* gnu/packages/glib.scm (itstool): Do as announced in 9000d556.

gnu: itstool: Add propagated inputs python-2 and libxml2.

* gnu/packages/glib.scm (itstool): Add propagated input python-2; it is needed
    to rewrite the shebang in the output python script and must be propagated
    so that PYTHONPATH gets set when using itstool; and libxml2, imported as a
    module by the script.

gnu: Add libcanberra.

* gnu/packages/libcanberra.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.

gnu: libvorbis: Propagate input libogg.

* gnu/packages/oggvorbis.scm (libvorbis): Propagate input libogg, as
    stipulated by the pkg-config files.

gnu: gnome: Add libnotify.

* gnu/packages/gnome.scm (libnotify): New variable.

gnu: racket: Revert input from gtk+-3 to gtk+-2.

* gnu/packages/scheme.scm (racket): Revert input from gtk+-3 to gtk+-2.

gnu: gtk+: Propagate inputs.

* gnu/packages/gtk.scm (gtk+): Propagate inputs at-spi2-atk, libxi and
    libxinerama.

gnu: Revert inputs from gtk+-3 to gtk+-2.

* gnu/packages/cdrom.scm (dvdisaster): Replace input gtk+ by gtk+-2.
* gnu/packages/emacs.scm (emacs): Replace input gtk+ by gtk+-2.
* gnu/packages/geeqie.scm (geeqie): Replace input gtk+ by gtk+-2.
* gnu/packages/gkrellm.scm (gkrellm): Replace input gtk+ by gtk+-2.
* gnu/packages/mp3.scm (ripperx): Replace input gtk+ by gtk+-2.
* gnu/packages/scheme.scm: Drop duplicate inclusion of module gtk.

gnu: gtk+-2: Update to 2.24.21.

* gnu/packages/gtk.scm (gtk+-2): Update to 2.24.21, add inputs perl and
    python.