Extracted the API of the Translation component into symfony/api

Bug fix: no
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: partially #6129
Todo: -
License of the code: MIT
Documentation PR: TODO

This PR implements the change suggested in #6129 for the Translation component. I think we should apply this change component by component so that we don't end up with one massive PR.