made leaves depend on the trunks they spawn with to avoid decaying

rather than just any trunk being enough to stop just any leaves from decaying.
Re-tuned the leaf decay interval/chance values accordingly.  Changed a few
variables to avoid possible conflicts.  Added a setting to allow one to disable
the new leaf decay code.  Moved sapling growth code into its own file.  Minor
changes to comments here and there.  Got rid of simple trees list (made the
code do various checks in realtime instead, since it's just startup code
anyway).  Fixed messed-up crafting for jungle trees; condensed most of the
crafting code into main craft registry loop.  Mostly fixed broken aliases of
default jungletrees.