Preview Bug

Bug fixing in split screen:

- [x] test coverage
- [x] finish the code
- [x] gather feedback for my changes


| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Tests pass?   | none
| Fixed tickets | https://github.com/sulu-cmf/SuluContentBundle/issues/84
| Doc           | none