Turns out that items could be in an order that isn't the same as how
things look on the carousel, so this change ensures that for testing
purposes they are sorted by Y position.
Also uses the `CarouselItem.Visible` flag to ensure that we don't have
to wait for drawable fade transitions after a filter operation.
From a user's perspective, changing a sort / order mode shouldn't filter
away results, but we were doing this.
In terms of UX expectations, in stable this kind of scenario would
results in a group being added to the end of son select with "Not
ranked" or "Unknown". I think we should aim to match this eventually.
This covers the fail case of removing and adding items (see
https://github.com/ppy/osu/issues/21926) but also covers the proposed
forward implementation, which now considers `DateAdded` and `OnlineID`.
This was causing multiple issues with masking and sizing and really
didn't need to exist in the first place. Also not sure why the pool was
nested inside the scroll container, but it isn't any more. Probably for
the best.