Introduce WarheadArgs

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

OpenRA.Mods.Common/Projectiles/GravityBomb.cs

@@ -89,7 +89,8 @@ namespace OpenRA.Mods.Common.Projectiles
 			{
 				pos += new WVec(0, 0, args.PassiveTarget.Z - pos.Z);
 				world.AddFrameEndTask(w => w.Remove(this));
-				args.Weapon.Impact(Target.FromPos(pos), args.SourceActor, args.DamageModifiers);
+
+				args.Weapon.Impact(Target.FromPos(pos), new WarheadArgs(args));
 			}
 
 			if (anim != null)