Formalize state cleanup between scenes

[Logodaedalus]

Currently there are lots of state variables from playthrough initialization (scenes, etc) that don't get cleared out or reset between scenes. We need to formalize our approach and add in that functionality (was custom-coded in Emma's Journey and then ripped out when we pulled the library out).

1. We should add in syntax for state variables to persist between scenes or even between playthroughs
2. What should be the default? What is the least likely to cause subtle bugs that are hard for authors to figure out?