Make HitShape mandatory for damaging actors and refactor warheads.

* Adds support for linking Armor traits to HitShapes.
* Adds spread support to TargetDamageWarhead
* Removes ring-damage support from HealthPercentageDamage
* Removes IsValidAgainst check from DoImpact(Actor victim...) overload
  and instead lets warheads perform the check beforehand
  (to avoid HitShape look-ups on invalid targets).
* Reduces duplication and improves readability of Warhead implementations

OpenRA.Mods.Common/Traits/HitShape.cs

@@ -12,6 +12,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using OpenRA.Mods.Common.HitShapes;
+using OpenRA.Primitives;
 using OpenRA.Traits;
 
 namespace OpenRA.Mods.Common.Traits
@@ -25,6 +26,10 @@ namespace OpenRA.Mods.Common.Traits
 		[Desc("Create a targetable position at the center of each occupied cell. Stacks with TargetableOffsets.")]
 		public readonly bool UseTargetableCellsOffsets = false;
 
+		[Desc("Defines which Armor types apply when the actor receives damage to this HitShape.",
+			"If none specified, all armor types the actor has are valid.")]
+		public readonly BitSet<ArmorType> ArmorTypes = default(BitSet<ArmorType>);
+
 		[FieldLoader.LoadUsing("LoadShape")]
 		public readonly IHitShape Type;