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.
About the WormSign notification - I maintain that we don't want it to be OnWormSpawn, but OnWormSign. Meaning it should be played exactly like "Enemy unit sighted"/"Enemy base detected"/"Stealth unit detected". Whether OpenRA currently has such an announcement is another question.