Less abstraction is better. As far as I can tell, we don't need a custom
model for this. If there's any tracking to be done, it should be done
within `BeatmapCarousel`'s implementation (or a filter).
Basically, I don't want bindables in `CarouselItem`. It means there
needs to be a bind-unbind process on pooling. By moving these to the
drawable and just updating every frame, we can simplify things a lot.
This was pointed out as an issue in the osu!taiko editor, but actually
affects all rulesets. Has now been fixed everywhere.
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Closes https://github.com/ppy/osu/issues/31548.
osu!mania could arguable be consdiered "more correct" with the old
display, but I don't think it's a huge deal either way (subjective at
best).