Fix toolbox items spontaneously contracting after briefly losing hover

Reproduction scenario:

1. Hover a toolbox item
2. Unhover the item, but do not hover any other item (can be done by
   exiting the toolbox completely to the right)
3. Come back to the item hovered in step (1)
4. The item would spontaneously contract after a second

osu.Game/Overlays/SkinEditor/SkinComponentToolbox.cs

@@ -122,6 +122,7 @@ namespace osu.Game.Overlays.SkinEditor
 
             protected override bool OnHover(HoverEvent e)
             {
+                expandContractAction?.Cancel();
                 expandContractAction = Scheduler.AddDelayed(() =>
                 {
                     this.ResizeHeightTo(expanded_size, animation_duration, Easing.OutQuint);