KV331 fixes (from Andrew Mansour)

Update to LilyPond 2.16 and apply fixes per Andrew Mansour to match IMSLP source.

I am not entirely satisfied with this update. I could not seem to clear the MIDI warning regarding ambiguous crescendo in MIDI, which is typically caused by a missing dynamic mark. There is also a warning telling me that a volta bracket could not be closed. I suspect this is caused by changes required in 2.16 WRT alternative that have different measure lengths --- see repeat starting around bar 97 where alternative 1 is a half-measure and alternative 2 is a full measure. The final PDF looks fine.