Piano Roll Editor should display/allow-edit "off time", not "length"

The indirect expression of note cutoff by "length" (in the Piano Roll Editor) makes modifying "on Time" needlessly difficult, of necessity involving an inconsistent intermediate state and mental subtraction which can easily introduce error. The case of delaying the note onset (as in rolled chords and violin chord bowing) will almost never include preserving the length and pushing the "off-time" to over 1000 (> 1000 off-times are in fact quite useful, but almost never as the side-effect of an on-time adjustment). The proposed change will not change the content, appearance, or sound of any scores, but only the UI of the Piano Roll Editor. OnTime/OffTime is intuitive; OnTime/Length is not. The customization of appoggiature, only possible in the Piano Roll Editor, would be greatly simplified and made more robust as well.