Level navigator

I added single-step navigation arrows to switch between the levels. They are directly under the level-progress bar:
![navigator-buttons](https://cloud.githubusercontent.com/assets/1278210/2543200/c3faf494-b5ef-11e3-8ba8-21cb0b477b9b.PNG)
When the outermost level is active the correspondending arrow turns dark signalizing that you cannot move in this direction.

This solves Feature Request #35