Tracks > Wheels

I reworked the physics system which solves some problems and creates others.

The most exciting change (in my opinion) is the tank tracks! They fly off sometimes but whatcha gonna do?

With the new physics the excavator's pose is saved as a part of the checkpoint so the arm shouldn't get stuck inside of the ground any more. Which only helps if your treads are still attached :P

The bucket slides around now but I'll see what I can do about that later on.