Bdd given steps first stage

This is the result (one part) of the discussion about [ObjectManager #126](https://github.com/ezsystems/ezpublish-community/pull/126)

So what was done here:
- [x] Update sentences so there is no hidden identifier (avoiding magic methods)
- [x] Create "object manipulators" for Given sentences ( aka "GivenContexts" )
- [x] Add given sentences and implementation for ContentTypeGroups