Scene files now live in the project

No longer comically hard-coded. Multiple scene can be added, but only
one is active at one time. Double-clicking a scene in the project
browser makes it active. No scene is loaded at startup, currently.
Reworked project file structure slightly

To make it more clear what's in the root folder and what's unrelated to
the folder structure.
Default scene can now be set for a project

Automatically activated on load
Highlight default scene bold in project browser