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.

Any reason in add/edit form these are grouped and note here. If it doesn't matter then we should grouped these here as well. i.e. `getEntityTypeLabels(TRUE)`