[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
Actually @chirimoya and I thought of something like this: https://github.com/sulu-cmf/SuluProductBundle/blob/feature/product-formular/Product/ProductManager.php#L167

We also thought of extracting the `getProperty`-Method into an own abstract Manager, so we could use them in all Managers. Then we don't need stuff like this closures, `method_exists`-calls or switch-structures.