title: How to interrupt the window closing mechanism in scala swing
body: <p>I'm building a GUI with the <code>SimpleSwingApplication</code> trait in scala swing.
  What I want to do is to provide a mechanism on close, that asks the user (Yes,No,Cancel) if he didn't save the document yet. If the user hits Cancel the <code>Application</code> shouldn't close. But everything I tried so far with <code>MainFrame.close</code> and <code>closeOperation</code> didn't work.</p>
  
  <p>So how is this done in Scala Swing?</p>
  
  <p>I'm on Scala 2.9.</p>
  
  <p>Thanks in advance.</p>