title: How can I modify the queue in just_audio?
body: <p>I'm trying to make a music app using just_audio and audio_service and I'm stuck at implementing the <code>onAddQueueItem</code> function. I can't find a way to append/modify the queue on just_audio, only how to reload just_audio with a new playlist via <code>player.setAudioSource</code>.</p>
  <p>Am I meant to use <code>player.setAudioSource</code> to change the current queue or am I missing something?</p>