title: Open window (or dialog) to block main window without stopping further instructions
body: <p>I have a <code>MainWindow</code> that will with the click of the button do some calculations that might take a few seconds.
  During that time I would like to show a little dialog (or separate window) saying "Please wait..." in the middle of <code>MainWindow</code> that will fully block it, so the user is not able to do anything until this dialog is closed.
  I tried creating a window and opening it with <code>window.ShowDialog();</code> but that will of course not continue the instructions...</p>
  
  <p>Is there any other way that will work?</p>