[WIP] [RFC] Product translation (Prezent)

This is a second approach using Prezent, please look at issue #2018
@pjedrzejewski Sylius removes the child class mapping and adds it to the parent class, but the Prezent listener, being the relationship already defined, doesn't re-define it. Therefore you get the parent class pointing to the child class translation.
This can be solved in two ways:
1) by defining the ``one to many`` relation between ``entity`` and ``translation`` in the ``orm.xml`` files 
2) Moving away from Prezent and implement our translation listener (based on it). We already moved away from the Prezent bundle.