So, this adds no features, but it does ensure thread safety of the code base. It fixes #25

There are two areas this affects:

1. tview, like most UI toolkits, has constraints about modifying view elements. This is documented by tview. I've wrapped these as best I could.
2. The code is cavelier about sharing data between goroutines. There wasn't too much of this; it mainly affected the array Player.Queue, which was modified and read in different goroutines. This code adds control via an RWMutex. I considered, and rejected, accessors; there few enough similar uses to seem worth it.

It's probably rare for any users to hit any of these. I put this in while trying to debug a different issue, which it didn't solve... but this change makes the code safer, so...