Unified diff tab added, see issue #436
Added save button #436
change font to monospace
Further development of file saving, including better filenames.
adding ly code highlighter in diff tab
diff highlighting
[rewrite xml export] basic handling of notes and rests
Merge pull request #453 from PeterBjuhr/convert-ly-diff

Convert ly diff
[rewrite xml export] bar attributes
[rewrite xml export] accidentals
[rewrite xml export] file structure
[rewrite xml export] inner structure
[rewrite xml export] scaler/tuplets
[rewrite xml export] scaling
[rewrite xml export] chords
[rewrite xml export] repeats
[rewrite xml export] voices
[rewrite xml export] grace notes and pianostaff/grandstaff
[rewrite xml export] VoiceSeparator construct
[rewrite xml export] tremolos