title: Change Flex Button Icon Alpha using skin
body: <p>I have a custom skin for spark buttons.</p>
  
  <p>I need to control the alpha of the image i set as the button icon using the skin. i.e. I need the image to change its opacity with change in the button states.</p>
  
  <p>Is there a workaround, using the skin class, rather than writing event handlers??</p>