PSThar - Tileset support

One user was running a PSThar script to migrate some tile properties. She noticed that the PSThar console wouldn't open when only a tile palette was visible, which is to be expected since PSThar gets registered as a level tool. I definitely see how that could be inconvenient, though. So today I made PSThar also register a tileset tool, called with the same hotkey. When the console is started on a tileset, a $CurTileset variable is set instead of $CurLevel.