Add panel field 'width' attribute

[RWDevelopment]

according to #671 (comment) pull request

<div class="section-content">
   <div class="row">
      <?php foreach ($fields->getMultiple($section->get('fields', [])) as $field) : ?>
         <?php if ($field->isVisible()) : ?>
             <div <?= $this->attr(['class' => ['col-md-'.$field->get('width', '12-12')]]) ?>>
                   <?php $this->insert('fields.' . $field->type(), ['field' => $field]) ?>
              </div>
         <?php endif ?>
      <?php endforeach ?>
   </div>
</div>

published:
        type: checkbox
        label: '{{page.status.published}}'
        description: '{{page.status.published.description}}'
        default: true
        width: 6-12

routable:
        type: checkbox
        label: '{{page.routable}}'
        description: '{{page.routable.description}}'
        default: true
        width: 6-12

[giuscris]

Thank you @RWDevelopment for the contribution

I've just fixed the spaces