Fix replacement of default trees so that it works correctly. Beeches
now use default tree nodes, but L-systems models. This redefines the
default tree, wood, leaves, and saplings so that they use moretrees'
textures "Beech" in their descriptions, but only if replacement of
default trees is enabled.
Added backward-compat aliases for this change.
Moved tree replacement mapgen->air aliases to a more logical spot in the
code.
Also, changed out all leaf decay dig_node() calls for nodeupdate(), since the
former calls the latter anyway, and the latter is what does the job.
Increased the palm leaf decay radius to 12.
This code will slow the map generator down a little due to bugs therein, and is
generally still in the experimental stages, but it works and is actually quite
fast!
Most of the added code written by RealBadAngel
Also adds a couple of additional biome controls to jungle trees and conifers.
Also adds a text file describing the biome settings in a human-readable manner.
At present, this doesn't actually add anything new, it just refactors the
code to allow for more trees, and adds a bunch of biome definitions, tree
models, textures, etc. but no code to use them, yet.
