[MusicXML export] Invalid MusicXML will be generated if exported sheet contains tied cue notes

Reproduction:
To reproduce this issue, one may export the uploaded sheet to MusicXML, generated MusicXML is invalid as it does not satisfy XSD of MusicXML.

Workaround?:
Users could workaround this by delete the disallowed < tie> element in affected < note>s. It is pretty annoying though.

Reason:
MusicXML does not allow a cue note has < tie> child element. The reason is that < tie> is designed for sound information, but cue notes are silent. Simple have a < tied> element under < notations> will be enough.

Possible Fix:
The fix should consists of a few lines of codes only. Correct logic will be, when parsing a tied note, does not add < tie> element into < note> when this note is grace and issue will be resolved.