Bootstrap ButtonGroup clientEvent not being assigned properly.

[yii-bot]

This issue has originally been reported by @alagodich at yiisoft/yii2#8028.
Moved here by @cebe.

---

Hello

As stated here: http://getbootstrap.com/javascript/#dropdowns-events

All dropdown events are fired at the .dropdown-menu's parent element.

But parent container for widget is being rendered via Html::beginTag and we cannot assign any events to it with 'clientEvents' option.

        return implode("\n", [
            Html::beginTag($tag, $this->containerOptions),