[Feature request] Add FSRS stats D, S and R to current card stats in sidebar

[galantra]

There are a couple of stats on the card info screen that cannot be selected in ARBb yet, namely the FSRS stats Stability, Difficulty and Retrievability. (Preset is another one.)

Also, when FSRS is enabled and Ease is selected in ARBb, it still shows it in the old SM-2 style for the current card, whereas in the card info screen and the review log Ease is replaced by Difficulty. I think the option in ARBb should be called Ease/Difficulty and show the former if FSRS is off and the latter if FSRS is on.

[noobj2]

Hey, I'll start working on it in a couple of months.