- make sure we set flag in changes when config var changed
- move current deck get/set into backend
- set_config() now returns a bool indicating whether a change was
made, so other operations can be gated off it
- active decks generation is deferred until sched.reset()
- .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.
