intro walkthrough

There are things that still need to be improved, but it should be ready for merging into master. The walkthrough can be entered from the splash page, and it leads the user through four steps on navigating, and working with points, areas and polygons.

things still left:
- after finishing all steps, it just jumps straight into editing. It should be clearer whats happening
- bottom bar that indicates progress could be styled a lot better
- no indication on how to save. Is this necessary? If yes, how detailed? just highlighting the button, or going through the process?
- re-running intro at any time


Original issue in #175