Do the few remaining things that GDK is doing for us:
- Open and close the X11 Display
- Set up a GSource on the Display FD to handle events
- Allocate and free the content of XGenericEventCookie,
to "unroll" the few XInput2 events that Mutter still
does handle.
And remove the GdkDisplay we've so long relied on.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2864>