Feature: Local music library

[bleonard252]

This PR introduces a new setting that allows you to set zero or more "local libraries" from a directory. These do not get modified in any way, not even created if they don't exist (they're just not loaded, in that case). Closes #595 (which seems to already be supported, actually).

This is only used right now in the "Local tracks" feature. Playback, queuing, removing libraries, and album art seem to work automatically, but MPRIS metadata does not, and it uses fallback data if you attempt to look at the track, album, or artist pages for any local track, but this seems to be an issue with the existing Downloads folder too.

The PR also changes some dependencies, which I had to do to get it to build and run on my machine. Feel free to revert it, or have me revert it, if you feel it necessary.

Screenshots