[FEATURE] Changed structure extensions to serializable data arrays in structure

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => https://github.com/sulu-cmf/SuluTestBundle/pull/19
| Fixed tickets | https://github.com/sulu-cmf/SuluContentBundle/issues/159
| BC Breaks     | changed interface to get extension data (described in Issue)
| Doc           | TODO