* gnu/services/colord.scm, gnu/services/dbus.scm,
gnu/services/upower.scm: Remove.
* gnu/services/desktop.scm: New file, with contents taken from the above
files.
* gnu-system.am (GNU_SYSTEM_MODULES): Adjust accordingly.
* doc/guix.texi (Desktop Services): New section.
(Various Services): Move colord-service and upower-service from
here to "Desktop Services".
