add Category to tileset entries

OpenRA.FileFormats/Map/TileSet.cs

@@ -36,6 +36,7 @@ namespace OpenRA.FileFormats
 		public string Image;
 		public int2 Size;
 		public bool PickAny;
+		public string Category;
 
 		[FieldLoader.LoadUsing( "LoadTiles" )]
 		public Dictionary<byte, string> Tiles = new Dictionary<byte, string>();