[playlists] Support opening M3U file outside of app's playlist directory

[basharovV]

Currently only m3u files in the playlists directory are scanned, and the tracks need to already exist in the songs table. It would be nice to:

• add a file association for any .m3u files to open in the app
• allow to listen to m3u playlist without importing tracks
  Note: Editing playlists (re-order, add, delete) writes changes back to the m3u file.