Add autocomplete for entity selection

https://www.drupal.org/node/2360725
> Or we could actually implement access checks on the buttons by falling back on the filter formats/editors that have them enabled?

Not sure if I understand what you mean with this. Using simiar approach that we use for dialog sounds ok, since autocomplete always comes in package with it. :)

Another thing crossed my mind. I guess we should also perform some kind of access check against entities before returning them?