fix(LayoutShadowNode): Ensure that `onLayout` event fires
fix(LayoutShadowNode): Ensure that `onLayout` event fires

For the onLayout event to fire, the UIManager must set the `onLayout` prop to true. This changeset adds a ReactProperty handler for this to the LayoutShadowNode base class.

Fixes #297

lbochenek/react-native-windows,lbochenek/react-native-windows,lbochenek/react-native-windows,lbochenek/react-native-windows