Add full support for editing GTK Menubar items
Currently in Glade there is only partial support for editing Menubars and their items.
You cannot create your own GtkMenuItem from the widget selection boxes instead you have to copy one of the existing items that Glade put in as a default and then paste it into where you need it. This currently seems to be a bad user experience in comparison to the level of support other widgets have, where users are free to create as many widgets of any kind and place them into their UI from Glade.
Currently the only ways to make Menus in GTK are to either construct it in code or compose your own XML definition by hand.