MusicXML: Fix problem with lyrics on notes with grace notes

MusicXML: Fix problem with lyrics on notes with grace notes

When a note had grace notes (and it was tied or part of a slur), sometimes
the lyrics for that note were completely ignored and all subsequent
lyrics were one note off. Now I simply ignore all slurs/ties of grace notes
and everything works just fine.

Also, clean up the staff change code. Now a staff change is not only applied
before a note, but also before other stuff (like clef change).