Flex Accordion header 图标实现方法
Accordion 导航头部加入图标:
1,复杂的 头部，可以有其他控件 ＋图标
http://dougmccune.com/blog/2007/09/18/using-complex-headers-with-the-flex-accordion/
2,mxml 实现 自定义 header renderer
http://blog.flexexamples.com/2007/09/24/creating-a-simple-flex-accordion-inline-header-renderer/
3,as实现 自定义 header renderer
http://www.kristoferjoseph.com/blog/2008/11/06/positioning-the-flex-accordion-header-icon
