[button] define the placement of icon (iconPlacement) in skin [ButtonSkin]

I think it would be better to define the placement of the icon in the button via Skin. Currently, we set iconPlacement (left, right, bottom, top). I realized that the iconDisplay is a SkinPart since Flex 4.5, but I can't see it in the default ButtonSkin. And the layout of the icon, label and custom skin-parts is wrong when I use the "icon" and a Custom Skin for the Spark Button / ToggleButton.

Why isn't the iconDisplay in the ButtonSkin?