Improved spotify integration

[KraXen72]

Describe the solution you'd like
Hi! I had an idea for improving the spotify integration:

• improved spotify integration
  • separate spotify page
  • ability to download new songs from playlist after it's been changed
  • ability to sync playlist with spotify
    • if song filenames are numbered & playlists download to a separate folder, delete & re-download / only download missing & re-number
    • otherwise, re-link existing songs (resolve in library), download new ones and re-create m3u playlist.
  • generate a .m3u file pointing to relative songs

@bambanah, Is this something you'd merge / want in the app?
This is something I'm even interested in eventually contributing (partially or fully), but no ETA, so if someone wants to do it before me, feel free to!

Describe alternatives you've considered
current implementation where spotify playlists are mixed with deezer ones in the favorites tab

Additional context
this would make for quite a convenient integration.