Fix dimmed checked nub artifact

This adds transition that extends nub's border to fill it.
Fill fade can be removed, but combined effect looks nicer imo,
and the fill is still needed because if removed, border
becomes invisible for some reason.

osu.Game/Graphics/UserInterface/Nub.cs

@@ -47,6 +47,7 @@ namespace osu.Game.Graphics.UserInterface
             };
 
             Current.ValueChanged += filled => fill.FadeTo(filled.NewValue ? 1 : 0, 200, Easing.OutQuint);
+            Current.ValueChanged += filled => this.TransformTo(nameof(BorderThickness), filled.NewValue ? 7 : border_width, 200, Easing.OutQuint);
         }
 
         [BackgroundDependencyLoader]