[WIP] [FEATURE] Reorder node manually

Implemented orderBefore method in content mapper:

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation
- [x] order-beofre in content-mapper
- [ ] UI (ContentBundle ...)

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/124
| BC Breaks     | no
| Doc           | TODO