Fixes DJ schedule bug, begins refactoring.

The DJ wasn't re-scheduling because the code was clearing the station's
playlist queue *after* it queued up the DJ. Oops.

Starts refactoring AudioClip to support temporary backLayers. Starts
refactoring game scene to make UI manipulation better.
Fixes bugs, adds functionality.

- Fixes a bug that made switching stations "tune out" the new station.
- Increases scheduled DJ talk horizon from 3s to 10s.
- Introduces "once layers" for audio clips. They can be cleared
    separately from regular permanent layers.
- Adds station IDs?