Add group support to beatmap carousel v2

[peppy]

This adds the basics of grouping – which is to say difficulty grouping.

Important changes:

• 645c26c I split out keyboard and group traversal into two methods because this reduces the complexity (less conditional pathing)
• d74939e Removed IsGroupSelectionTarget from CarouselItem. This is no longer required and is instead handled using CheckValidForGroupSelection. Less memory overhead and less complexity (basically allows behaviour to be implemented for traversal of groups without even more bool flags).

2025-02-03.02.32.23.mp4

What's next?

• Expanded state for groups and sets (required for display purposes, mainly). Probably IsVisible becomes a ternary enum { Hidden, Contracted, Expanded }.
• Grouping with nested sets (ie. artist / title / collection grouping). Should in theory work with no changes.

[bdach]

isn't this already checked in the outermost conditional? (line 80)

[peppy]

Removed this.

[bdach]

Not sure how much to nitpick if at all over input handling, so please cut me off if this is out of scope for now, but this seems like a possibly unintended behaviour:

2025-02-03.13-18-07.mp4

When a group header is keyboard-preselected, attempting to navigate to previous or next group toggles the expanded state of the group instead.

[peppy]

When a group header is keyboard-preselected, attempting to navigate to previous or next group toggles the expanded state of the group instead.

I've attempted to fix this (and added test coverage), please give it another go and also check whether the code still makes logical sense. The important change was to actually confirm that keyboard selection transfer actually worked, and if it doesn't continue with normal logic.

[bdach]

Seems ok to go forward with