Feature/dom structure

__Tasks:__

- [x] create a three column layout
- [x] build a nice api to change the column width behavior from maximum to fixed
- [x] create a sidebar component which is in the right most column
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | no => https://github.com/sulu-cmf/sulu/pull/123
| Fixed tickets | none
| Doc           | TODO