More realistic MIDI for Rachmaninoff Op.23/2, and more cleanup

MIDI is a bit closer to real performance with hidden tempo, dynamics and
hairpins on each staff, as well as redefined MIDI volume.
Globally replace accent with smaller version.
Editorial changes added to file top section (in addition to existing
inline comments) to explain differences between editions.
Compress accidentals on certain bars so it doesn't look too crowded.
Even more curve tuning.
Show/hide fingering function now replaced by tags.
Remove all fingerings from 2nd occurrence of main theme.