Keys for lyrics services

Applies to nori 0.6 on Android, the desktop and the terminal.

Two services take a key of your own, entered in Settings, Lyrics:

Key	What it adds
PaxSenix key	PaxSenix’s Musixmatch and Spotify lyrics. Without it, those two are skipped quietly.
BetterLyrics key	lyrics for songs BetterLyrics has not stored yet. Without one it answers only for songs it already has.

Keys are kept with your settings on the device and sent only to their own service.