In GtkSpriteInterface, attached menu item callback to the ButtonPressEvent event as well as the Activated event for items in sub-menus. This is because the activated event does not fire until the sub-menu has focus which is fairly useless as clicking the menu will cause it to disappear without doing anything. By attaching to both events, sub menu items will work when clicking or activated via keyboard. We do not attach to both events for top-level items or else the callback will get fired twice.