Biome API / dungeons: Add biome-defined dungeon nodes

Biome API / dungeons: Add biome-defined dungeon nodes

Add new biome fields 'node_dungeon', 'node_dungeon_alt', 'node_dungeon_stair'.
If 'node_dungeon' is not defined dungeons fall back to classic behaviour.

Remove messy and imprecise dungeon material code from 'generateBiomes()'.
Code deciding dungeon materials is now in 'generateDungeons()' and uses the
biome at mapchunk centre for more precision.

Remove hardcoded 'MG_STONE' types as long intended.