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).
With the animation logic being external, it was going to make it very
hard to apply the scroll offset when a new panel is added or removed
before the current selection.
There's no real reason for the animations to be local to beatmap
carousel. If there's a usage in the future where the animation is to
change, we can add more customisation to `Carousel` itself.
Also:
- Adds hinting at grouping and header status of items
- Passes through criteria and prepare for grouping tests.
- Makes `Filters` list `protected` because naming clash with `Filter()`
on `BeatmapCarousel`.
