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.
You can augment https://github.com/OpenRA/OpenRA/blob/bleed/OpenRA.Mods.Common/Traits/Sound/AnnounceOnKill.cs in a similar fashion to play a notification instead of a unit voice.