IOS-111: STKQueueMixer now supports play/stop/pause. Added -(void)playNext:withID:duration:fadeAt to allow an entry to jump the queue and be played as soon as the currently playing item finishes. This will put the currently primed entry back on the queue at the front, ready to play when the high priority item has finished. Note that if the next up item is in the process of being faded in, it will still be replaced with the high priority item.
IOS-111: Remove lingering audio artefacts following stop/play of queue.
IOS-111: Output volume can now be set.