code: func (v *ToolItem) SetProxyMenuItem(menuItemId string, menuItem IMenuItem) {
  	cstr := C.CString(menuItemId)
  	defer C.free(unsafe.Pointer(cstr))
  	C.gtk_tool_item_set_proxy_menu_item(v.native(), (*C.gchar)(cstr),
  		C.toGtkWidget(unsafe.Pointer(menuItem.toMenuItem())))
  }
docs: // SetProxyMenuItem is a wrapper around gtk_tool_item_set_proxy_menu_item().