[NUI] Add OnBackNavigation to Navigator

[NUI] Add OnBackNavigation to Navigator

To enable customization of back navigation, OnBackNavigation is added
to Navigator.
e.g.
class CustomNavigator : Navigator
{
    protected override OnBackNavigation(BackNavigationEventArgs args)
    {
        // Do something when back navigation is started.
        ...

        // To pop the peek page, call base.OnBackNavigation().
        // Otherwise, do not call base.OnBackNavigation().
        base.OnBackNavigation(args);
    }
}