Statistics calculation

[Dan Meltzer]

Hi there

Currently Track::finishedPlaying is being passed inaccurate values on
track change.  We call trackPosition() after currentSourceChanged is
emittedwhich results in an unpredictable result.  The solution is to
assume that if aboutToFinish is emitted that the track was played
completely and to do partial song calculations in the playlist code
instead.

Unfortunately my laptop shat another brick before I could fix this so
if someone else could take a look at this it would be appreciated.