glib/menu.go
func (v *MenuItem) SetSubmenu(submenu *MenuModel) {
	C.g_menu_item_set_submenu(v.native(), submenu.native())
}
func (v *MenuItem) SetSubmenu(submenu *MenuModel) {
	C.g_menu_item_set_submenu(v.native(), submenu.native())
}
//SetSubmenu is a wrapper around g_menu_item_set_submenu().