* Empty cards is undoable
If there was a reason for this operation not to be undoable, I can't easily guess it. My main hyposhesis was that the number of deleted card may be too big. But I realized that deleting a deck is undoable and may delete as many note.
As you may know, I realized that only the undoable operations triggered notification in AnkiDroid that we may have to update the UI. And while I just wanted to trigger more notifications, some reviewers thought it would be nicer if the operation were returning a OpChanges. So here it's done. If you would please consider merging it.
I decided to introduce a new string because the closest strings I could find currently are "Empty cards..." and the trailing commas don't seem nice in "undo". And the title, which we may not be able to reuse in all language
* Don't count cards that have already been removed (dae)
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