Added support for Shadow Behavior in NativeViewHost

[ichan-mb]

Experimental support for drawing Shadow inside NativeViewHost.

Test Case:

<App Background="White">
	<ClientPanel Color="White">
		<StackPanel Alignment="Center" ItemSpacing="20">
			<Rectangle Size="200" Color="Red" StrokeColor="Blue" StrokeWidth="1" CornerRadius="10" ux:Name="rect">
				<Shadow Size="10" Distance="5" Color="Green" />
			</Rectangle>
			<NativeViewHost>
				<Rectangle Size="200" Color="Red" StrokeColor="Blue" StrokeWidth="1" CornerRadius="10" ux:Name="rect2">
					<Shadow Size="10" Distance="5" Color="Green" />
				</Rectangle>
			</NativeViewHost>
		</StackPanel>
	</ClientPanel>
</App>

Note:
Shadow drawing (distance, size) slightly different between in graphics view and nativeviewhost

This PR contains:

• Changelog
• Documentation
• Tests

[mortend]

• https://travis-ci.org/github/fuse-open/fuselibs/builds/743722638