[FEATURE] Added Navigation Context config to webspace xml

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation
- [x] xsd adaptions
- [x] dump navigation contexts
- [x] adopt valiate nav contexts
- [x] update ui (https://github.com/sulu-cmf/SuluContentBundle/pull/229)

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => https://github.com/sulu-cmf/SuluTestBundle/pull/24
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/206 , https://github.com/sulu-cmf/SuluContentBundle/issues/188
| BC Breaks     | none
| Doc           | none