Add the default item renderers to the spark manifest

There will be a new default item renderer for mobile use and it will be added to a manifest file so it can be declared in MXML. This way it is easy for developers to tweak properties on that item renderer without having to create a whole new class, for example:

<s:List id="testCaseList" labelField="name" ...>
<s:itemRenderer>
<fx:Component>
<m:IconItemRenderer iconWidth="40" iconHeight="40" />
</fx:Component>
</s:itemRenderer>
</s:List>

Similarly the current default item renderers (DefaultItemRenderer and DefaultComplexItemRenderer) should be added to the spark manifest so developers can make slight tweaks on those renderers as well. For example if someone wanted to use (the nicely optimized) DefaultItemRenderer but change the width and height slightly.