Experiment with caching music grouping

[OxygenCobalt]

Experiment if caching the music processor/grouper results would be a good idea. Not going to move the whole app to SQLLite, just want to see if storing and reading them at load-time would be a good idea.

It's possible that it might speed up some things on huge libraries, especially if I actually just use a binary format that can be serialized into the music graph in O(n) time.