Make it easier to get to the Xamarin.Forms control from the Blazor component

[Eilon]

You can now do this to access the native Xamarin.Forms control

<StackLayout>
    <Label Text="@("You have pressed " + counter + " times")" />
    <Button @ref="incrementButton" Text="+1" OnClick="HandleClick" />
</StackLayout>

@code {
    Microsoft.MobileBlazorBindings.Elements.Button incrementButton;

    int counter;

    async Task HandleClick()
    {
        counter++;

        await incrementButton.NativeControl.RotateTo(360);
        incrementButton.NativeControl.Rotation = 0; // Reset rotation
    }
}

The NativeControl property on a given Blazor component is strongly-typed to its target Xamarin.Forms type. You can then call methods, set properties, etc.

A word of caution: Changing state in this manner is not tracked by Blazor, which can cause problems. For example, if you set the Text of a Label control in this manner, it will mess up Blazor's behavior. Use this only when you need to alter some native UI state, such as controlling an animation, or calling methods that affect some more global state.

Fixes #39

[Eilon]

cc @davidortinau @jamesmontemagno

[Eilon]

Docs are live here: https://docs.microsoft.com/mobile-blazor-bindings/advanced/access-native-controls