Improved Scrolling during playback

[Richard Shann]

While doing playback in the main window the scrolling has always been
difficult to follow. 
I've fixed this now so that the currently playing bar is kept in the
same place on the screen as the music plays. You can adjust the
position of currently-playing bar while it is playing as well, then it
keeps the new position in the screen you have chosen. Tempo changes are
handled as well.

Richard