refactor(GraceNotes): remove OpenGraceBeam, handle as OpenBeam
refactor(GraceNotes): remove OpenGraceBeam, handle as OpenBeam

remove special case OpenGraceBeam

part of #56

opensheetmusicdisplay/opensheetmusicdisplay,opensheetmusicdisplay/opensheetmusicdisplay,opensheetmusicdisplay/opensheetmusicdisplay