Provide ViewContract property on ViewModelViewHost for XAML platforms

[kentcb]

This PR fixes #870 by adding a ViewContract property (of type string) to the ViewModelViewHost implementation on XAML platforms. The idea is to simplify use cases such as:

<ListBox x:Name="listBox">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <rxui:ViewModelViewHost ViewContract="listitem" ViewModel="{Binding .}"/>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

Previously this would require some pretty ugly hackery to get the appropriate view contract in use by the ViewModelViewHost.

NOTE: I only implemented this for XAML platforms because it seems of higher importance there. Also, I'm not really set up to build other platforms yet so I didn't really want to risk committing broken code.

[anaisbetts]

Thanks!

[anaisbetts]

/cc #850