window: Add "is_focus_async" API

X11 clients can use different models of input handling, of which some
may not result focus being set synchronously.

For such clients, meta_focus_window() will not change the focus itself
but rely on the client itself to set the input focus on the desired
window.

Add a new MetaWindow API to check when dealing with such a window.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1716>

src/core/window.c

@@ -8631,6 +8631,12 @@ meta_window_is_stackable (MetaWindow *window)
   return META_WINDOW_GET_CLASS (window)->is_stackable (window);
 }
 
+gboolean
+meta_window_is_focus_async (MetaWindow *window)
+{
+  return META_WINDOW_GET_CLASS (window)->is_focus_async (window);
+}
+
 MetaStackLayer
 meta_window_calculate_layer (MetaWindow *window)
 {