If menu_item_id matches the string passed to set_proxy_menu_item() return the corresponding GtkMenuItem.
Custom subclasses of GtkToolItem should use this function to update their menu item when the GtkToolItem changes. That the menu_item_ids must match ensures that a GtkToolItem will not inadvertently change a menu item that they did not create.
See also: retrieve_proxy_menu_item() , set_proxy_menu_item()