Custom Warheads refactor

Changes included:

Warhead code split out of weapon code and refactored.
Warhead functionality now split into several classes, each handling one effect/impact.

Additional custom warheads can now be defined and called via yaml.
Custom warheads inherit the abstract class Warhead,
which provides target check functions.

Custom warheads have to define their own impact functions,
and can also define their own replacement for check
functions.

OpenRA.Mods.RA/Invulnerable.cs

@@ -18,6 +18,6 @@ namespace OpenRA.Mods.RA
 
 	class Invulnerable : IDamageModifier
 	{
-		public float GetDamageModifier(Actor attacker, WarheadInfo warhead) { return 0.0f; }
+		public float GetDamageModifier(Actor attacker, DamageWarhead warhead) { return 0.0f; }
 	}
 }