[MusicXML Import] Adding option in import dialog. Fix issue #464
musicxml2ly has a midi-block option on import:
-m, --midi activate midi-block
This is implemented as default and with the new checkbox you can turn it off (comment out midi block).
Merge pull request #466 from PeterBjuhr/musicxml

[MusicXML Import] Adding option in import dialog. Fix issue #464