We setup Xwayland in an early phase of the X11 display, before we had a
MetaX11Display, and teared down in a couple of places happening when
tearing down the Xwayland integration if the X server died or
terminated. It was a bit hard to follow what happened and when it
happened. Attempt to clean this up a bit, with things being structured
as follows:
* Early during X11 display connection setup, only setup the rudimentary
X11 hooks, being the libX11 error callbacks, and adding the local
user to XHost.
* Move "initialize Xwayland component" code to a new
'x11-display-setup' signal handler. Things setup here are cleaned up
in the 'x11-display-closing' handler.
* Connect to 'x11-display-setup' and 'x11-display-closing' up front,
and stay connected to these two.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1796>