Swing Playback implementation using staff texts

Swing playback using staff texts implemented, facilitating local control over swing parameters. Also added tests for the same.
That is the sort of thing that makes me a little nervous about using floating point values.  In this case, I would hope it is safe.  He's grabbing it directly from the Style value, and the only way that ever gets set is by the radio buttons that set it explicitly.  There may well be *other* places where it's not so clear, though.