D2k: Sandworms implementation

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.
This `AttackRequiresEnteringCell` check should really be applied inside `HasAnyValidWeapons` (the weapon isn't valid if the actor can't move into the cell).  That would make this additional check unnecessary.