Accidentals xml

This should fix #17508 and it should take care of software compatibility. I tested the export/import against a demo version of Sibelius 7.5.
Because of the way Sibelius exports microtonal accidentals to MusicXml, I had to partially rewrite my previous PR (which in the meantime got merged): sorry for the previous almost superfluous PR. It seems that Sibelius rounds the "alter" value to integer, determining (in the previous PR) a wrong import in MuseScore of 3/4 tone accidentals. Now it should work as expected.
I couldn't install the Dolet plugin (it does not seem to work with a demo), so I don't know if it exports better MusicXml files.
If someone owns a copy of Finale and/or Sibelius with Dolet, could she/he please test the import/export? (Actually, in the Sibelius Demo I could only enter 1/4 and 3/4 tone accidentals; these are now properly handled)