wayland/pointer: Use glib signals tracking focus surface

Use the "destroy" MetaWaylandSurface signal instead of the wl_resource
destroy signal for tracking the lifetime of the surface with pointer
focus.

As unsetting the focus may have side effects due to handlers of the
"focus-surface-changed" signal, connect the signal after the default
handler to make sure other clean up facilities have the chance deal with
the surface destruction before we try to unset the focus.

https://bugzilla.gnome.org/show_bug.cgi?id=783113

src/wayland/meta-wayland-pointer.h

@@ -71,7 +71,7 @@ struct _MetaWaylandPointer
   GHashTable *pointer_clients;
 
   MetaWaylandSurface *focus_surface;
-  struct wl_listener focus_surface_listener;
+  gulong focus_surface_destroyed_handler_id;
   guint32 focus_serial;
   guint32 click_serial;