title: Add icon on spark button skin
body: <p>I am trying to add a different icon each of my buttons. I have my skin file ready but not sure if I have to create a different skin class for each button. It sounds inefficient. Any suggestions?</p>
  
  <pre><code>&lt;s:Button id="pass"
      width="110"
      height="35"
      fontWeight="bold" 
      fontSize="12" 
      fontFamily="arial" 
      label="Past Track" 
      click="pass_clickHandler(event)" 
      skinClass="skins.CustomSkin"/&gt;      
  &lt;s:Button id="future" 
      width="110"
      height="20"
      fontWeight="bold" 
      fontSize="12" 
      fontFamily="arial" 
      label="Future Plan"
      click="future_clickHandler(event)"
      skinClass="skins.CustomSkin"/&gt;
  </code></pre>
  
  <p>The skin:</p>
  
  <pre><code>&lt;!-- layer 2: fill --&gt;
  &lt;!--- @private --&gt;
  &lt;s:Rect id="fill" left="1" right="1" top="1" bottom="1" radiusX="2"&gt;
      &lt;s:fill&gt;
          &lt;s:LinearGradient rotation="90"&gt;
              &lt;s:GradientEntry color="#304fd7" 
                               color.over="#4b6bf6" 
                               color.down="0xAAAAAA" 
                               alpha="0.85" /&gt;
              &lt;s:GradientEntry color="#1f38a3" 
                               color.over="#3653cf" 
                               color.down="0x929496" 
                               alpha="0.85" /&gt;
          &lt;/s:LinearGradient&gt;
      &lt;/s:fill&gt;
  &lt;/s:Rect&gt;
  
  &lt;!-- icon --&gt; // I could add my icon here but that would make me create  
                // a different icon image for different button
  </code></pre>