Add a potion of gain level to the Castle chest

This is partly for balance reasons (so that clearing the Castle
helps towards the level 14 Quest unlock) and partly as a clue to
players spoiled on previous versions that the Castle wand now works
differently.

dat/castle.lua

@@ -144,6 +144,7 @@ local loc = place:rndcoord(1);
 des.object({ id = "chest", trapped = 0, locked = 1, coord = loc ,
              contents = function()
                 des.object("wishing");
+                des.object("potion of gain level");
              end
 });
 -- Prevent monsters from eating it.  (@'s never eat objects)