Introduce WarheadArgs

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

OpenRA.Mods.Common/Projectiles/Bullet.cs

@@ -280,7 +280,7 @@ namespace OpenRA.Mods.Common.Projectiles
 
 			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));
 		}
 
 		bool AnyValidTargetsInRadius(World world, WPos pos, WDist radius, Actor firedBy, bool checkTargetType)