Bundle Tracker 3 in the Flatpak for systems which don't have it

An easy way to test the tracker3 version of Music!

This depends on !728 (closed) and provides a way to run the app without requiring tracker3 on the host, as proposed here.

Current issues:

• Music displays the "Tracker is unavailable" screen while it is in fact waiting for the bundled tracker-miner-fs-3 to start. The needs changing in the UI.
• Notifications don't appear to work -- adding/removing files from my ~/Music folder doesn't trigger updates in the Music UI. (maybe an issue with !728 (closed) too)
• I saw an MP3 file identified as text/plain by tracker-extract -- gio info showed the correct MIME type, so i'm mystified what happened here. Maybe it's a harmless issue.