Added an in-game map editor

I polished my ingame-map-editor branch for #3377 to allow the workflow of creating a blank new map, editing the terrain tiles and saving the map. Next is actor placement (at least spawn-/waypoints) and modifying the resource layer.
It is actually weird. When you set the facing to 0 one turret will vanish:

![image](https://cloud.githubusercontent.com/assets/756669/5564941/47a8b08c-8edb-11e4-92fd-a8987df5ddc5.png)