Use the 'editor' display builder for admin form

This is a follow-up to #2340999: Implement "display renderer" plugin type (now called "display builder")

Currently, we're just showing a table with the names of the regions on the layout. It would be great to demonstrate the layout on the admin page like in Drupal 7!

To do that, let's build out the 'editor' display builder and use that for the admin form (same architecture as in Drupal 7).