Auto translate popup menus of MenuButton and OptionButton

Currently, `auto_translate` property of a `MenuButton` or `OptionButton`
won't affect its popup menu. It's okay if you want to auto translate,
but requires extra setup if you don't.

scene/gui/menu_button.cpp

@@ -169,6 +169,10 @@ void MenuButton::_notification(int p_what) {
 				menu_btn_other->get_popup()->set_focused_item(-1);
 			}
 		} break;
+
+		case NOTIFICATION_TRANSLATION_CHANGED: {
+			popup->set_auto_translate(is_auto_translating());
+		} break;
 	}
 }