Switch auxio to a single-queue system. "Play next" adds songs to the top
of the queue, similar to before, and then "Add to queue" adds songs to
the bottom of the queue. This enables many more enhancements to be made
to the playback experience, at the cost of a feature I preferred.
Resolves#44.
Re-add the ability for content to fade out as the playback or queue
view slides up. This also migrates the queue slide animation to also
fade out content, which allows for the stylistic cohesion I desire.
