Add unique entity validator
After creating a unique entity validator for Neos I shared it in the IRC channel and people seemed to like the idea. So I've moved it into the Flow package and would like some feedback if people would like to have this general validator or if we should keep it out.
It works for action controllers and is intended for create and update methods. The uniqueness is defined by the entity's identity properties. It should work with both Doctrine and Generic persistence.
Thanks to RafaelK, sorenmalling & afoeder for helping getting it to the final implementation.