preview template edit-toolbar

added preview template to edit-toolbar:
merge after: https://github.com/sulu-cmf/SuluContentBundle/pull/21

- [x] test coverage
- [x] finish the code
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Tests pass?   | none
| Fixed tickets | no
| Doc           | none