Img skins can now scale using aspect ratio (centered) and crop to fill space

Center background image skin within widget, centered, whilst maintaining aspect ratio:

    <Widget
        width="200"
        height="200"
        skin:Img-src="'image.png'"
        skin:Img-scaleImg="true"
        skin:Img-keepAspect="true"
    >

Fill entire widget with proportionally correct image. Crop off edges to fill space. Good for scaling background images:

    <Widget
        widthPt="100"
        heightPt="100"
        skin:Img-src="'background.png'"
        skin:Img-scaleImg="true"
        skin:Img-keepAspect="true"
        skin:Img-crop="true"
    >