App: Control the metronome playback from the lockscreen

The app could treat the metronome as media playback. This would allow you to play and pause the metronome and switch between songs of a setlist, without even unlocking the phone, right from the lock screen On the negative side, this would mean listening to a song e.g. on Spotify while playing the metronome would no longer be possible. We could enable an app setting that allows you to toggle this behavior on and off, so if you want you could disable it and still play music while running the metronome.

Changed the title from "App (iOS): Control the metronome playback from the lockscreen" to "App: Control the metronome playback from the lockscreen"