[NUI] Add OnBackNavigation to Page

[NUI] Add OnBackNavigation to Page

To enable customization of back navigation per page, OnBackNavigation
is added to Page.
e.g.
class CustomPage : Page
{
    protected override OnBackNavigation(PageBackNavigationEventArgs args)
    {
        // Do something when back navigation is started.
        ...

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