[WIP] [FEATURE] added additional tabs (seo, abstract)

implemented seo, abstract api, structure extension and ui:

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation
- [ ] structure extension (https://github.com/sulu-cmf/sulu/pull/120)
- [ ] seo extension
- [ ] abstract extension
- [ ] seo api
- [ ] abstract api
- [ ] ui refactoring
- [ ] ui seo and abstract


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | yes (refactoring, redirect for non existing nodes)
| New feature?  | yes
| Tests pass?   | no => https://github.com/sulu-cmf/sulu/pull/120
| Fixed tickets | none
| Doc           | url to commit