fix MusicXml import of natural-flat and natural-sharp accidentals
fix import for quarter tone and three quarters tone accidentals when accidental tag is missing
fix microtonal accidentals export
fix microtonal accidentals import (e.g. from a MusicXml file exported by Sibelius 7.5)
cleanup and comments
Merge pull request #1173 from AntonioBL/accidentals_xml

Accidentals xml