Introduce WarheadArgs

- Passes additional arguments to warheads on impact
- Uses that to reduce parameter count of DoImpact by 1

OpenRA.Mods.Common/Activities/Air/FallToEarth.cs

@@ -41,7 +41,7 @@ namespace OpenRA.Mods.Common.Activities
 				if (info.ExplosionWeapon != null)
 				{
 					// Use .FromPos since this actor is killed. Cannot use Target.FromActor
-					info.ExplosionWeapon.Impact(Target.FromPos(self.CenterPosition), self, Enumerable.Empty<int>());
+					info.ExplosionWeapon.Impact(Target.FromPos(self.CenterPosition), self);
 				}
 
 				self.Kill(self);