code: protected function initDefaultButtons()
      {
          if (!isset($this->buttons['collapse'])) {
              $this->buttons['collapse'] = function ($option) {
                  $option['class'] = 'btn '.$option['class'];
                  $option['data-widget'] = 'collapse';
                  $option['data-toggle'] = 'tooltip';
                  $option['title'] = Yii::t('yii', 'Collapse');
                  
                  if($option['notitle']){
                      unset($option['notitle'],$option['title']);
                  }
                  
                  return Html::tag('button','<i class="fa fa-minus"></i>', $option);
              };
          }
          if (!isset($this->buttons['remove'])) {
              $this->buttons['remove'] = function ($option) {
                  $option['class'] = 'btn '.$option['class'];
                  $option['data-widget'] = 'remove';
                  $option['data-toggle'] = 'tooltip';
                  $option['title'] = Yii::t('yii', 'Remove');
                  
                  if($option['notitle']){
                      unset($option['notitle'],$option['title']);
                  }
                  return Html::tag('button','<i class="fa fa-times"></i>', $option);
              };
          }        
      }
docs: Initializes the default button rendering callbacks