services: desktop: Switch to GDM.

* gnu/services/desktop.scm (%desktop-services): Replace
SLIM-SERVICE-TYPE instance with an instance of GDM-SERVICE-TYPE.
* doc/guix.texi (Keyboard Layout): Change example to mention
GDM-SERVICE-TYPE.
(X Window): Mention GDM.
(Desktop Services): Adjust references to SLiM.