Add a collection operation to unbury cards

@dae:

In aqt/operations/scheduling.py there’s no function to unbury a selection of cards though there are similar functions for suspending/unsuspending.

After consulting the new manual section and the .45 porting notes I tried to write my own unbury cards operation for the fastbar add-on with this commit. I’d prefer to use official code in such a popular add-on and using an official function would be more future proof. I can imagine that some other people with future add-ons would also benefit from such a function. So maybe Anki could carry such a function even if it’s not needed for Anki itself at the moment.

Or did I miss something and needlessly create new code?

You didn’t miss anything; I’ll add this in the next beta

Thanks for your help.