add OnPlayQueueChange hook

backend/playbackmanager.go

@@ -74,6 +74,11 @@ func (p *PlaybackManager) OnVolumeChange(cb func(int)) {
 	p.engine.onVolumeChange = append(p.engine.onVolumeChange, cb)
 }
 
+// Registers a callback that is notified whenever the play queue changes.
+func (p *PlaybackManager) OnQueueChange(cb func()) {
+	p.engine.onQueueChange = append(p.engine.onQueueChange, cb)
+}
+
 // Registers a callback that is notified whenever the player has been seeked.
 func (p *PlaybackManager) OnSeek(cb func()) {
 	p.engine.onSeek = append(p.engine.onSeek, cb)