Add Support for Three Voces Per Stave

modifiercontext.js :: recode formatNotes to handle three voices. Add
auto note shift and/or auto stem shortening.

note.js :: fix bug when voice is right shifted.

stavenote.js :: add functions to support three voices

vex.js :: add function to sort a structure array by any of it's
elements.

textx/flow.html :: add three voice tests

tests/threevoice_tests.js :: new three voice tests