[WIP][FEATURE] Handling product sets as products

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | [yes|no]
| New feature?  | [yes|no]
| Tests pass?   | [yes|no => <WHY?>]
| Fixed tickets | [<WHICH?>|none]
| Doc           | https://github.com/sulu-cmf/docs/commit/8d59bd4d1eeed743c4358bfc89f72289beb400f5