title: Check if a ContentDialog is open or not
body: <p>I have a simple question. Since I can only have one <code>COntentDialog</code> open at a time and in the closing event of <code>ContentDialog</code> I have a condition that prevents the <code>ContentDialog</code> from closing by doing <code>args.Cancel = true</code>. So doing <code>ContentDialog.Hide()</code> can sometimes not close the <code>ContentDialog</code> and when opening a dialog again will cause an exception. 
  Therefore, before any dialog, I want to be able to check if other dialogs are closed. How can I do that ?</p>