Initial implementation of Sandworms (WIP; Need help with audio and animations)

Using Mailaender's code as basis, this is my implementation of the sandworm behavior. It is still WIP, but you can get an idea of how it looks from the shellmap.
Changed to self, but now reverting to subject for readability. 
The self here:

    public override Activity Tick(Actor self)

is now the world actor, since I changed the way worms and their targets are removed from the world.