Asset support for named cells/frames

Frames can now either be specified numerically as before or now with names. Example:

```xml
<ImageAsset  
    AssetName="soldierWalk"  
    ImageFile="@assetFile=soldierWalk.png">    
    <ImageAsset.Cells>  
        <Cell RegionName="walk1" Offset="2 2" Width="103" Height="66"/>  
        <Cell RegionName="walk2" Offset="179 100" Width="37" Height="35"/>  
        <Cell RegionName="walk3" Offset="2 70" Width="65" Height="31"/>  
        <Cell RegionName="walk4" Offset="161 137" Width="36" Height="41"/>          
    </ImageAsset.Cells>  
</ImageAsset>  
  
<AnimationAsset  
    AssetName="soldierWalkAnim"  
    Image="@asset=ToyAssets:soldierWalk  
    NamedAnimationFrames="walk1 walk2 walk3 walk4"  
    AnimationTime="1" /> 
```