[FEATURE] Added Navigation Context config to webspace xml

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