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.
Remove this whole dummy trait with the unused requires and notification string. Create a new modular trait instead:

```csharp
using OpenRA.Traits;

namespace OpenRA.Mods.D2k
{
	[Desc("Play a speech notification when the sandworm enters the world.")]
	public class AnnounceOnWormSpawnInfo : ITraitInfo
	{
		public object Create(ActorInitializer init) { return new AnnounceOnWormSpawn(init.self); }
	}

	public class AnnounceOnWormSpawn
	{
		public AnnounceOnWormSpawn(Actor self)
		{
			Sound.PlayVoice("WormSign", self, self.World.LocalPlayer.Country.Race);
		}
	}
}
```

Disclaimer: I have not tested this.