title: Start a new line in wpf textbox
body: <p>I created a small GUI with WPF, containing a Textbox.
  I want the user to have the ability to start a new line .
  How do I let the user start a new line in the WPF textbox (with \n or \r)? 
  I want the user to be able to start a new line by pressing "enter".</p>