[WIP][FEATURE] introduced api entities

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | -
| Doc           | -

__TODO:__
- issues for full implementation of api entities?

__Requires:__
- https://github.com/sulu-cmf/SuluMediaBundle/pull/63
- https://github.com/sulu-cmf/SuluCategoryBundle/pull/20
- https://github.com/sulu-cmf/SuluTagBundle/pull/42
I am not sure if I would extract these values... They are just an implementation detail, that should not be relevant for the user of the objects.