[FEATURE] added additional tabs (seo, abstract)

implemented seo, abstract api, structure extension and ui:

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation
- [x] structure extension (https://github.com/sulu-cmf/sulu/pull/120)
- [x] seo extension
- [x] abstract extension
- [x] seo api
- [x] abstract api
- [x] ui refactoring
- [x] ui seo and abstract
- [x] seo data in twig template (https://github.com/sulu-cmf/sulu-standard/pull/98)

Other dependencies: https://github.com/sulu-cmf/SuluAdminBundle/pull/125

__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           | https://github.com/sulu-cmf/SuluContentBundle/issues/138
@wachterjohannes What is this property needed for?