[FEATURE] implemented structure extensions

implemented structure extensions:

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | https://github.com/sulu-cmf/SuluContentBundle/issues/138