Avoid the X11 backend on g-s-d daemons
A bunch of things happened that made this possible in the mean time. As of currently, and gsd-xsettings aside, the only usages of x11 are:
- ICC profile
- X screensaver watchdog
- X bell settings
The first one has already checks for Wayland. This MR fixes the last 2 to do the same (they're pretty much useless on Wayland, too), and make all g-s-d daemons stop requesting the x11 gdk backend specifically.