gtk/gtk.go
func (v *MenuItem) SetSubmenu(submenu IWidget) {
	C.gtk_menu_item_set_submenu(v.native(), submenu.toWidget())
}
func (v *MenuItem) SetSubmenu(submenu IWidget) {
	C.gtk_menu_item_set_submenu(v.native(), submenu.toWidget())
}
// SetSubmenu() is a wrapper around gtk_menu_item_set_submenu().