windowManager: Implement MetaDisplay::init-xserver hook

windowManager: Implement MetaDisplay::init-xserver hook

We do spawn gsd-xsettings, and watch its name before notifying on the
given task, so the mutter bits can proceed with X11 startup.

One notable change is that we only start gsd-xsettings, instead of the
generic gnome-session-x11-services target. We do so as we have to wait
on a dbus name to appear in order to deem the initialization done, and
making it all depend on gsd-xsettings seems tidier.

Less notably, we also use ::shutdown-xserver to shutdown the related
services. Its major benefit is that it'd allow us to ensure the olderly
shutdown of those services, but it's unused at the moment.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/836