MusicXML: Add support for longa and breve notes, fix error messages

MusicXML: Add support for longa and breve notes, fix error messages

-) Longa and breve notes have a duration_log < 0, so I can't use
   1<<duration_log and write that out as the duration in lilypond.
	 Instead, I need to check for negative durations manually and
	 output \longa or \breve for these notes!
-) Update the musicxml regression test to also check longa and breve
   notes (and their dotted versions)
-) Correctly import lilylib as ly (in particular, since the
   error messages used ly.stderr_write already!!!!)

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