title: show a page inside a window in XAML
body: <p>I have a tabcontrol in my window, inside each tabitem I want to have a different page.<br>
  I can achieve this by making a Frame inside the TabItem and in the behind code use for example:</p>
  
  <pre><code>frame1.Content = new Pages.MyPage()  
  </code></pre>
  
  <p>How can I do the same thing in XAML?</p>