attempted to maintain BC as good as possible, but RegistryInterface::getRepository() had to be dropped from RegistryInterface. Its still part of the ManagerRegistry, so its only a BC break for people using RegistryInterface to create their own implementation
* added a RegistryInterface
* changed all classes to depend on the Registry instead of a specific EntityManager
This is more consistent as the validator already took the registry and this allows
to use any entity manager in Forms.