Multiple buttons

d.o issue: https://www.drupal.org/node/2273361

Note: This is just the first step in order to support different types of entities for selection. Read my last comment (short essay: https://www.drupal.org/node/2273361#comment-8999597) on the issue for more details.
I don't think this is needed for `$this->entityManager()` you can remove it and inject the service. It'll be good for unit testing.