MusicXML: Fix regression when adding expressions like octave shifts

MusicXML: Fix regression when adding expressions like octave shifts

Octave shifts in lilypond (like in MusicXML) are not associated with a
particular note but their position in the measure. So far, I simply
inserted them like a note, but unfortunately that reset the current
starting position in the measure (needed to find the correct position
for chords). So I added a method dedicated to inserting such commands
without resetting the position (of course, pending multi-bar rests
still need to be inserted before the music command).

Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>