Implement library rescan logic and "force rescan" option

[JoshStrobl]

This is a TODO for implementing library rescanning logic and "force rescan" option. This should leverage a couple mechanisms:

1. Filesystem Notifications
   a. Library directory FS notify events (useful for when we are adding or removing artists, podcasts, etc.)
   b. Per-artist directory FS notify events (useful for when we are adding or removing albums)
   c. Per-album directory FS notify events (useful for monitor track changes so we can proactively update our KotoIndexedTrack and database references)
2. "Manual" recursive rescan of a library dependent on Cleanup and separate indexer logic. #10.