Entity embed config entity

d.o issue: https://www.drupal.org/node/2273361

@todo:
- Support for customizing button icons.
- ~~Remove entity type field from embed form.~~ [Done]
- Add CRUD tests.

OK, automatic upcasting is working fine now (like it should have earlier also). I think there was something else which I did wrong before. But in any case, this is working now.