use the ajax framework
put our custom command class in the Ajax namespace
Attach the Drupal.ajax object to the embed element itself
oops, downcast() operates on htmlParser.element, no removeAttribute() method there
blacklist a couple behaviors
assign ID in upcast()
Merge branch '8.x-1.x' into use_ajax_framework
Merge pull request #103 from yched/use_ajax_framework

Previewing an entity in the WYSIWYG does not add any attached styles or javascript from the AJAX callback