MetaWaylandPointerConstraint: Use own 'appears-focused' signal handler

Instead of having MetaWindowWayland having hooks into pointer
constraints subsystem, have the pointer constraints subsystem listen
for the signal itself and enable/disable itself.

https://bugzilla.gnome.org/show_bug.cgi?id=762661

src/wayland/meta-window-wayland.c

@@ -403,11 +403,6 @@ appears_focused_changed (GObject    *object,
                          gpointer    user_data)
 {
   MetaWindow *window = META_WINDOW (object);
-  MetaWaylandCompositor *wayland_compositor;
-
-  wayland_compositor = meta_wayland_compositor_get_default ();
-  meta_wayland_pointer_constraint_maybe_remove_for_seat (wayland_compositor->seat,
-                                                         window);
 
   /* When we're unmanaging, we remove focus from the window,
    * causing this to fire. Don't do anything in that case. */
@@ -415,8 +410,6 @@ appears_focused_changed (GObject    *object,
     return;
 
   surface_state_changed (window);
-
-  meta_wayland_pointer_constraint_maybe_enable_for_window (window);
 }
 
 static void