window: Move UI frame event handler to a separate function

In order to tell whether an event has been handled by the frame UI, move
the UI frame handler to a specific helper function.

https://gitlab.gnome.org/GNOME/mutter/issues/88

src/core/window-private.h

@@ -782,6 +782,9 @@ void meta_window_handle_enter (MetaWindow  *window,
                                guint        root_y);
 void meta_window_handle_leave (MetaWindow  *window);
 
+gboolean meta_window_handle_ui_frame_event (MetaWindow         *window,
+                                            const ClutterEvent *event);
+
 void meta_window_handle_ungrabbed_event (MetaWindow         *window,
                                          const ClutterEvent *event);