Improve layout system

[objorke]

As described in comment on #1

[jvlppm]

Setting Platform and IsPlatformEnabled on models, trigger the automatic layout system under Xamarin.Forms, requiring renderers to use element.Bounds to position child elements. When those properties are set, calling rootElement.Layout(bounds) will position all elements according to Xamarin.Forms.Core implementation. With this change StackLayoutRenderer is not needed, and Grid will work as expected, without specific implementation.