Usability: Voxel scenes

[heyx3]

1. Define the process for adding new scene files, and loading them
2. Make it fault-tolerant, by inserting an "error" scene if the normal scene fails to load
3. Integrate into the GUI:
   a. Report the available scenes. A "live preview" would probably be pointless though, since that's what this entire program is meant to do!
   b. Allow the user to select/load a scene.

[heyx3]

I think the "error" scene is a bad idea; users will want their current scene to persist if they make an error typing (or if the compiler starts too early and compiles a half-typed scene).