MusicXML: Implement rehearsal marks and some more spanners

MusicXML: Implement rehearsal marks and some more spanners

-) Implement dashes (\startTextSpan and \stopTextSpan) and bracket (although
   I'll still need to add the bracket engraver, since I use \startGroup and
	 \stopGroup, which does not automatically print the bracket)
-) Pedal change is now implemented as \sustainUp\sustainDown
-) Add MarkEvent and MusicGlyphMarkEvent to convert rehearsal marks and
   segno/coda markups. Rehearsal marks can also be boxed (default) or circled
-) Revamp the way directions are converted. Now they also use a dict like the
   notations, where one can give a general conversion function, or a finished
	 class