Added product categories for flat list (group concatenated)

__Tasks:__

- [x] gather feedback for my changes

https://github.com/sulu-cmf/sulu-complete/pull/8
https://github.com/sulu-cmf/sulu/pull/318

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => https://github.com/sulu-cmf/SuluProductBundle/issues/45
| Fixed tickets | none
| BC Breaks     | no
| Doc           | none