title: How to switch to another xaml file in the same window?
body: <p>I have 2 xaml files L1.xaml and L2.xaml 
  At first L1 will open. It has a button which when pressed opens L2.
  But I want L2 to open in the same window as L1. </p>
  
  <p>More precisely, when the button is clicked, L1 should terminate and L2 should start without the main window disappearing even for a second.</p>