Adapting to new ly.music design for durations

The duration node is now placed as a child in the tree structure, but only if it's present in the source code.

The duration type etc of the current note is now set with the value of the last note and changed afterwards if a duration child is found.
Unfold repeats directly
Clear ly dependency earlier

Keep object structure MusicXML generic.
More adjustments to the object structure
Adjustment to rest with position
Use copy function instead
Adjustments for chords
Reference pitch must be absolute
More cleaning up of the duration code
Preserve numerator and denominator for scaler elements
Create the new bar first when multiplying rests
Merge pull request #507 from PeterBjuhr/musicxml

Musicxml Export wip