allow custom icons on units

OpenRa.Game/Chrome.cs

@@ -57,7 +57,7 @@ namespace OpenRa.Game
 				.Where(u => Rules.UnitInfo[u].TechLevel != -1)
 				.ToDictionary(
 					u => u,
-					u => SpriteSheetBuilder.LoadAllSprites(u + "icon")[0]);
+					u => SpriteSheetBuilder.LoadAllSprites(Rules.UnitInfo[u].Icon ?? (u + "icon"))[0]);
 
 			tabSprites = groups.Select(
 				(g, i) => Pair.New(g,

OpenRa.Game/GameRules/UnitInfo.cs

@@ -51,6 +51,7 @@ namespace OpenRa.Game.GameRules
 		public readonly int FireDelay = 0;
 		public readonly string LongDesc = null;
 		public readonly int OrePips = 0;
+		public readonly string Icon = null;
 
 		public UnitInfo(string name) { Name = name; }
 	}