MusicXMl: Fix fretboard assignment to voices.

MusicXMl: Fix fretboard assignment to voices.

Instead of assigning <harmony> elements to every voice or only the very
first voice (in the frist case, we'll have duplicated frets, in the
second one the fretboards will often be inserted at the completely
wrong position), simply insert the fretboard into the next note that
we encounter in the MusicXML file.

I suppose we could also do something similar for text markup (i.e.
<direction><words>...</word></direction>), but there we need to
distinguish direction elements with words-only subelements and with
dynamics subelements.