Removed QueueStatic

Instead, just pass the Player instance that owns the ZPlayer instance.
Replaced Next/Previous with ChangeSong

Mostly for the actual engines -- because Next() and Previous() are
pretty much identical, other than which song they're going to change to
Reworked EngineQueue

It didn't make any sense before - we were peeking instead of dequeueing
and vice versa, it seemed like.  Now, I changed things around a little
and it seems to be working fine - also, added a History to the queue
that we can use for Previous().