MusicXML: Implement Barline styles and repeats in musicxml2ly

MusicXML: Implement Barline styles and repeats in musicxml2ly

-) The <barline> element is now converted to a \bar"...." lilypond
   expression. Not all bar styles from MusicXML are supported by lilypond,
   though.

-) Convert repeats and alternative endings from MusicXML to lilypond.
   While lilypond has a nice hierarchy (i.e. nested music) for repeating
   structures, MusicXML only has markers "here starts/ends a repeat or
   altern.ending", so I need to somehow build up that repeat hierarchy
   from the flat data. I've imlemented this via a loop that looks for
   a repeat structure, replaces that one repeat by a proper instance
   of musicexp.RepeatedMusic. Since I directly modify the list of
   event chords, after replacing one repeat, I have to start the
   whole loop from the start (because the iterators are off after
   the list is modified).

-) Add some more unit test files for barlines and repeating structures
   (some even do not make sense from a musical point of view)

-) Fix typo in the coverage/regression tests make file