Tony: a tool for melody transcription

Justin writes in a comment to #874:

I think we need to make a distinction between the "original" curve and the "current" curve. The original is the one returned by pYin prior to any editing. The current is the pitch curve in its current state (potentially after editing), but BEFORE the user entered "candidate mode". My point is, when the user hits Esc (assuming we implement this), they probably want to revert to the "current" curve, not to the "original" curve (think of a case where the user is editing overlapping parts of the curve for some reason).

This seems very sensible I think. That is,

• If you make a selection, change the current pitch candidate within it, and then hide the pitch candidates with Ctrl+Return, your change should remain in force, but
• If you make a selection, change the current pitch candidate within it, and then abandon the selection by hitting Esc, your change should be reverted to whatever it was before you made the selection