Add an "Autosync Tempo" feature. This uses a linear Least Squares

Regression to figure out a line through the user's step errors to minimize
that error.  It then uses the slope of that line to fix the tempo and the
intercept to fix the offset.  Stops are adjusted as if they were originally
calculated as a number of beats, ie the slope is also used to change
the stops.

Also fixed is a bug where the user doesn't get a chance to keep or
reject the sync changes when ESCing (ZZing) from a song.

R=glenn

stepmania/src/ScreenGameplay.cpp

@@ -2245,6 +2245,7 @@ void ScreenGameplay::SaveStats()
 
 void ScreenGameplay::SongFinished()
 {
+	AdjustSync::HandleSongEnd();
 	SaveStats(); // Let subclasses save the stats.
 	/* Extremely important: if we don't remove attacks before moving on to the next
 	 * screen, they'll still be turned on eventually. */