Start/stop game BGM depending on whether Spotify is playing music

[RhapsodySL]

Currently switching between music in-game and from Spotify/FantasyPlayer is a two step process, where you need to turn off the game music to listen to Spotify and vice versa. FantasyPlayer already implicitly detects when Spotify is playing music, so it'd be neat if it had an option to mute the BGM when Spotify is playing, and unmute the BGM when it's no longer playing music.