add defferred generation callback mechanism, to generate specific
features after all of another feature has finished generating (e.g.
generate lamps AFTER all structures are built)
add minimum Y level of (64-7) for town-generation, explicit exception
for superflat world types.  Might cause issues with non-overworld
dimensions having non-standard surface heights (twilight forest?)
hopefully a proper fix for world-type detection, using getHorizon() to
determine world height.
fix detection of improper target block types; exit out if an improper
block is detected during town area validation
enable generation of random villagers in a town template.  Uses
best-attempt to find a valid generation spot for each villager;  tests
up to 10 random positions within the chunk to attempt to find a valid
one.
fix crash bug with instant structure builder when click on air