Add Display plugin options

d.o issue: https://drupal.org/node/2283691

When this issue goes through, following follow-up issues will be needed:

* Remove redundant 'view-mode' field from the form.
* Use rebuildEmbedForm() in submitForm() when form['state'] is select.
I think this could be massively simplified by using the actual AJAX framework to only replace part of the form that needs to be updated (the `$form['attributes']['data-entity-embed-settings']` element). See https://api.drupal.org/api/drupal/core!includes!ajax.inc/group/ajax/8