Added IDisabledTrait & rewrote upgrade code using a level-based approach.

Upgradeable traits are notified whenever an upgrade of their declared types are granted or revoked.  The traits maintain their own internal level counter, which is then used to enable or disable the trait functionality.  A trait can register for multiple upgrade types which then all affect the internal level counter.

	IDisabledTrait for identifying (and filtering) disabled traits
	UpgradableTrait provides an abstract base for traits to support upgrade levels
	Added IDisabledTrait support to GlobalButtonOrderGenerator

	Includes rework by pchote with alterations.

OpenRA.Mods.RA/InvulnerabilityUpgrade.cs

@@ -11,41 +11,24 @@
 using System;
 using System.Collections.Generic;
 using OpenRA.GameRules;
+using OpenRA.Mods.Common;
 using OpenRA.Traits;
 
 namespace OpenRA.Mods.RA
 {
-	public class InvulnerabilityUpgradeInfo : ITraitInfo
+	public class InvulnerabilityUpgradeInfo : UpgradableTraitInfo, ITraitInfo
 	{
-		public readonly string RequiresUpgrade = "invulnerability";
-
 		public object Create(ActorInitializer init) { return new InvulnerabilityUpgrade(this); }
 	}
 
-	public class InvulnerabilityUpgrade : IUpgradable, IDamageModifier
+	public class InvulnerabilityUpgrade : UpgradableTrait<InvulnerabilityUpgradeInfo>, IDamageModifier
 	{
-		readonly InvulnerabilityUpgradeInfo info;
-		bool enabled;
-
 		public InvulnerabilityUpgrade(InvulnerabilityUpgradeInfo info)
-		{
-			this.info = info;
-		}
-
-		public bool AcceptsUpgrade(string type)
-		{
-			return type == info.RequiresUpgrade;
-		}
-
-		public void UpgradeAvailable(Actor self, string type, bool available)
-		{
-			if (type == info.RequiresUpgrade)
-				enabled = available;
-		}
+			: base (info) { }
 
 		public int GetDamageModifier(Actor attacker, DamageWarhead warhead)
 		{
-			return enabled ? 0 : 100;
+			return IsTraitDisabled ? 100 : 0;
 		}
 	}
 }