Make the button labels for apply / cancel of edits more consistent

Before you could 'Cancel' the edit and then would be asked if you wanted
to 'Discard' your changes or 'Cancel'. So clicking 'Cancel' cancelled the
action of having clicked 'Cancel'. That's so confusing, it's even hard to
explain. Yes, it uses "typical" language for user interaction and kind of
makes sense, but it's not easy to understand for the non-technical user.

With the new labels the user is asked to whether they want to 'Apply' the
changes or 'Discard' them. And when they choose 'Discard' the verification
question is still 'Discard' or 'Cancel'. That seems much more consistent.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>