[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 I think that's not even necessary, since the changer, which is a `User`-object, should always have this method. @alexander-schranz Did you maybe introduce it for the tests? Then we should maybe add `getFullName` to the `TestUser`.