title: Xamarin.Forms Add NavigationBar
body: <p>I want to manage the nav bar appearence in the xaml?</p>
  
  <p>And how can I manage the nav bar appearence in a Tabbed Page?
  NavigationBar.SetHasNavigationBar(tabPage,true) seems not working.</p>
  
  <p>Thank you all,</p>
  
  <p>Ilenia </p>