menu: notify attach-widget property when menu is detached

When the menu is detached, the attach-widget property changes value to
NULL, so we should notify a property change, like
gtk_menu_attach_to_widget() does.

https://bugzilla.gnome.org/show_bug.cgi?id=679454

gtk/gtkmenu.c

@@ -1297,6 +1297,7 @@ gtk_menu_detach (GtkMenu *menu)
   /* Fallback title for menu comes from attach widget */
   gtk_menu_update_title (menu);
 
+  g_object_notify (G_OBJECT (menu), "attach-widget");
   g_object_unref (menu);
 }