Refactor ModifierContext, move formatters for Dots, Accidentals,
FretHandFinger, GraceNoteGroup, and StaveNote into their respective
classes.
Moved Stroke, StringNumber, and Articulation modifier formatting into their
respective classes.
Refactored the rest of the modifier formatting. Also refactored
getCategory for all modifiers.
Create PREFORMATTERS and POSTFORMATTERS arrays to simplify modifier creation.
Merge remote-tracking branch 'upstream/master'