It appears that we were storing PyHSPlasma objects in class attributes, so
they were being held after the plResManager went away--therefore their
keys were nuked. We should probably be holding keys and not objects, but
this fix works regardless.
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.