In a previous changeset, the Advanced Logic modifier was changed to
defer exporting logic until the very end of the export process. This
means that many nodes are designed with the assumption that all
non-logic PRP objects are fully exported by the time they are exported.
LogicWiz modifiers, however, violated this assumption by exporting
during the main export() phase.
So, this adds a new `pre_export()` phase for all modifiers that lets
them generate logic trees and even entirely new Plasma Objects safely.
Futher, old LogicWiz modifiers have been tweaked to not leak junked
objects if the export fails in the middle of those modifiers.
We are attempting to encourage the pages=layers paradigm; however, this is
not strictly enforced. This commit has lots of bits and bobs but little
actual substance. More to come when I begin work on the exporter.