Change landmarks to use absolute voxels

Landmarks class holds locations in absolute voxels, rather than
relative.  Default landmark changed to Clearing. Chest placing fixed and
more landmark rendering added.
Add mcdungeon commands for Treasure Hunts

Add support for treasure hunts to the main interface.  Added 'addth'
command; added treasure hunts to 'interactive', 'list' and 'genpoi', as
well as to 'delete' and 'regenerate'.