title: Proper way to display pop-ups using WPF M-V-VM pattern
body: <p>I had a question about the VM responsibilities when it comes to pop-ups. 
  When an app is popping a message box or some kind of dialog (with MVVM), the two options that we have are:</p>
  
  <ol>
  <li>putting UI (ShowDialog()) code in VM which seems bad</li>
  <li>have VM send some kind of event that UI can subscribe to and display a dialog in the   code behind (but we are striving for zero code behind :) )</li>
  </ol>
  
  <p>How do you guys tackle this case?</p>