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:

• [x] Changelog
• [ ] Documentation
• [ ] Tests

[mortend]

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