Sort exported notes by step when same midi_pitch

[leleogere]

In MusicXML export, the notes are currently sorted using their midi_pitch attribute. However, this can lead to inconsistent exports when the score contains notes that are enharmonic equivalents (such as D# and Eb, or B# and C), as the export will differ depending on the order the user inserts the notes in the Part object:

score.add(pt.score.Note("C", 4, 0), start=0, end=1)  # midi_pitch=60
score.add(pt.score.Note("B", 3, 1), start=0, end=1)  # midi_pitch=60
# will result in a different file than
score.add(pt.score.Note("B", 3, 1), start=0, end=1)  # midi_pitch=60
score.add(pt.score.Note("C", 4, 0), start=0, end=1)  # midi_pitch=60

This PR simply adds the step as a second key for the sorting. Therefore, notes are first sorted by midi_pitch, and when they are enharmonic equivalents they are sorted by step, so that the export file is consistent no matter the order the notes are added to the score.

This will solve a potential issue raised in the second message of #406.

[fosfrancesco]

An effective solution for the inconsistent ordering of enharmonic notes in the musicxml export.