Add a preview for the selected entity in WYSIWYG

d.o issue: https://drupal.org/node/2280547
When I wrote this, I was assuming that preview will be generated only when someone uses our plugin to embed an entity and in which case both entity-id and entity-uuid will refer to same entity. But someone may also try to embed directly using source and in that case that assumption will not be valid. So, I'll add both cases here now. Good catch.