Rel 4.2 radiobutton checkbox enhancements

[pikausp]

This PR covers the proposals from #348 (comment) with the exception of FontFamily. I noticed the property was there already and I don't see a need for databinding.

I turned the Value property of RadioButton into bindable to make something like the following possible

<input:RadioButtonGroupView BindableLayout.ItemsSource="{Binding Items}"
                            BindableLayout.ItemTemplate="{StaticResource ItemTemplate}"
                            SelectedItem="{Binding SelectedItem, Mode=TwoWay}"/>

[enisn]

Perfectly done!

Thanks for your contribution 🎉