Enable use of "custom" palettes per tile Templates

Keep the tileset's palette as default, defined on the tileset, but override it for any tile templates that may want to do so with a palette defined on the template.

OpenRA.Game/Map/TileSet.cs

@@ -73,6 +73,7 @@ namespace OpenRA
 		public readonly int2 Size;
 		public readonly bool PickAny;
 		public readonly string Category;
+		public readonly string Palette;
 
 		readonly TerrainTileInfo[] tileInfo;