Magic Folder List / File Browser List: Make use of dividers

@florian used some CSS hackery to add a gray line between items in our lists.

Today I found out there's dividers for that:

  <li class="mdc-deprecated-list-item">
    <span class="mdc-deprecated-list-item__ripple"></span>
    <span class="mdc-deprecated-list-item__text">Item 2 - Division 1</span>
  </li>
  <li role="separator" class="mdc-deprecated-list-divider"></li>
  <li class="mdc-deprecated-list-item">
    <span class="mdc-deprecated-list-item__ripple"></span>
    <span class="mdc-deprecated-list-item__text">Item 1 - Division 2</span>
  </li>

Make use of them and rip our custom CSS.