- .update() should update a single deck and preserve usn by default,
as that's what existing code expects
- decks are automatically renamed when they conflict with an existing
name
The progress bar add-ons appear to be the only active users of it;
they can switch their old code from iterating over the list to
simply locating the selected deck in deckDueTree(), as its counts should
summarize all the child decks.
