Improvements to DungeonHelper

Removed hardcoded references to all our internal dungeons. The code will
be replaced by reading from a list in the next commit. Renamed
customDungeons to untaggedDungeons and removed registration of tagged
dungeons into that list. Now all tagged dungeons (custom and internal)
are listed in registeredDungeons. Fixed the comparator used to sort
dungeon names so that it sorts them case insensitively.
Fixed CommandCreateDungeonRift

Forgot to fix a reference to DungeonHelper in CommandCreateDungeonRift
during the last commit. Made some further improvements to DungeonHelper
to avoid exposing internal variables.