Add Three Voice Per Stave With Rest Repositioing

formatter.js :: Correct spelling error in comment.

modifiercontext.js :: Rewrite formatNotes to support three voices per
stave and automatic rest positioning to avoid collisions

note.js :: Correct doubling of note widths

stavenote.js :: Calculate rest bounding boxes. fix ledger lines for
rests above/below the stave, and new functions for setting stem lengths
and manually displacing notes.

tables.js :: Add glyph parameters for rest bounding box.

vex.js :: Correct spelling error in comment

flow.html :: add tests for three voices per stave formatting

threevoice_tests.js :: New tests for three voices per stave.