[FEATURE] refactor api entities with entity wrapper classes

__Tasks:__

- [x] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none
Erm, looks like a commented the wrong line, that comment applies to https://github.com/sulu-cmf/SuluMediaBundle/pull/21/files#diff-0579b72ea38af4231512cb9848fe6f7dR147