Rachmaninoff Prelude Op. 23/1

Mostly complete, pending some very minor tweaks and formalities. Midi is
probably beyond rescue though.
Allegro finished, before Meno Messo
Sync with Muzyka edition

Bring formatting more in line with Muzyka edition, instead of (inferior)
Gutheil edition
Merge branch 'rachmaninoff-prelude23' of https://github.com/abelcheung/The-Mutopia-Project
Great Gate, use two ily files
Fix some automatic layout glitches

Fixing easy hanging fruit like ugly slurs. Also move more hidden tempo
to dynamics section, as well as some dynamics to left hand.
Header tweaks
Add Rachmaninov
Recompile with Lilypond 2.19.7
Merge branch 'great-gate' of https://github.com/ksnortum/The-Mutopia-Project
Merge branch 'rachmaninoff-prelude23-01' of https://github.com/abelcheung/The-Mutopia-Project
Update Mutopia footer