MusicXML: Correctly handle bar checks with attributes and MM rests

MusicXML: Correctly handle bar checks with attributes and MM rests

-) Put the bar line BEFORE the attribute
-) Add Bar checks around MM rests (of course not inside MM rests!)
-) If we have a MM rest, globally set Score.skipBars=##t
-) Don't print a bar check at the very beginning of a score