Improved DungeonHelper

Completed changes so that our code uses registerDungeon() (formerly
registerCustomDungeons() ) to register all dungeons, both bundled with
the mod and custom ones. Made some changes to the code to hide
implementation details from other classes. Also, I had some problems
with old dungeons being mixed into the renamed ones. I had to remove
fallingTNThall from the schematics folder and deleted the WIP folder
because I think we have completed versions of those schematics already.

Some of our dungeons have pre-filled chests and the contents are more
generous than our current loot system (e.g. ruinsO has a prefilled
chest). I'll have to wipe all the chests later.