xwayland: Make setup/teardown a bit more symmetrical

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>

src/core/display.c

@@ -889,8 +889,12 @@ meta_display_new (MetaContext  *context,
 #ifdef HAVE_WAYLAND
   if (meta_is_wayland_compositor ())
     {
+      MetaWaylandCompositor *wayland_compositor =
+        meta_wayland_compositor_get_default ();
       MetaX11DisplayPolicy x11_display_policy;
 
+      meta_wayland_compositor_init_display (wayland_compositor, display);
+
       x11_display_policy = meta_context_get_x11_display_policy (context);
       if (x11_display_policy == META_X11_DISPLAY_POLICY_MANDATORY)
         {