Add support for card retirement after a given retirement interval

[yuki-tsubaki]

As the title implies, this is a feature request for simulating 'retirement' of cards by marking them as retired and removing them from the simulation once their interval grows to be greater than or equal to a given 'retirement interval'.

This allows simulating manual 'pruning' of the deck (by removing cards with long intervals) or, more accurately, the behavior of other Anki add-ons which automatically retire cards when they reach or surpass a set retirement interval.

I have already implemented this locally (updated the UI, tested, and ready to push). If this repo is still maintained, I will open a pull request. If not, I will either create my own fork (somewhat undesirable as I am likely not the most qualified Python UI developer out there) and add it there, or see if I can find a more qualified maintainer and add it to their fork.