Disallow interaction with carousel set difficulty icons unless selected

I kinda liked this flow, but from multiple reports from users it
definitely seems in the way. We can revisit after the new design is
applied to song select.

Note that this means the tooltips also don't display. If it is preferred
that they should (arguable from a UX perspective, since I'd expect to be
able to click at that point) then the issue can be addressed using a
slightly different path (a few more lines - nothing too complex).

osu.Game/Screens/Select/Carousel/SetPanelContent.cs

@@ -16,6 +16,9 @@ namespace osu.Game.Screens.Select.Carousel
 {
     public class SetPanelContent : CompositeDrawable
     {
+        // Disallow interacting with difficulty icons on a panel until the panel has been selected.
+        public override bool PropagatePositionalInputSubTree => carouselSet.State.Value == CarouselItemState.Selected;
+
         private readonly CarouselBeatmapSet carouselSet;
 
         public SetPanelContent(CarouselBeatmapSet carouselSet)