The handling of cleanup is performed only the `Item_Set` method. This
was already correctly called for `DrawableCarouselBeatmapSet`, but not
for the class in question here.
This would cause runaway memory usage at song select when opening many
beatmaps to show their difficulties. For simplicity, we don't yet pool
these (and generate the drawables each time a set is opened) which isn't
great but likely will be improved upon when we update the visual /
filtering of the carousel. But this simplicity caused the memory usage
to blow out until exiting back to the main menu when cleanup would
finally occur.
Reproduction steps:
1. Go to song select
2. Open beatmap listing
3. Import a beatmap that would fit the current filter criteria
4. The count of visible beatmaps does not change
Fixed by updating the count on `BeatmapSetsChanged` too.
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.