Local-file source layers

[stuartpb]

While this'd be nice, it's approaching the point where it'd be easier to just have a network of entities and operations that you visually build a chain of, due to the interface complexity involved in obtaining file system handles and local-system-dependent components in general.

It could be done by letting the user specify named local sources, and then have a dialogue for each of those, but it honestly starts to sound like more effort than it's worth versus just posting builds online and pulling them at runtime.