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/Armor.cs

@@ -11,6 +11,9 @@
 
 namespace OpenRA.Mods.Common.Traits
 {
+	// Type tag for armor type bits
+	public class ArmorType { }
+
 	[Desc("Used to define weapon efficiency modifiers with different percentages per Type.")]
 	public class ArmorInfo : ConditionalTraitInfo
 	{