added ActionColumn::$buttonOptions
added ActionColumn::$buttonOptions

allows for configuration of the default buttons like this:

```php
[
	'class' => ActionColumn::className(),
	'template' => '<div class="btn-group text-center">{view} {update} {delete}</div>',
	'buttonOptions' => [
		'class' => 'btn btn-default btn-xs',
	],
],
```

close #7633

yiisoft/yii2-framework,yiisoft/yii2-framework