title: Add click listener on on button in Ext.window.MessageBox
body: <p>I am creating dinamically a message box (Ext.window.MessageBox)</p>
  
  <pre><code>var msgBox = Ext.create('Ext.window.MessageBox', {
         title: '',
         //message in window
         msg: 'message text',
         icon: 'WARNING',
         //buttons
         buttons: 'OKCANCEL',
         //onclick funciton
         fn: myfunc
   });
  </code></pre>
  
  <p>I am adding OK and Cancel buttons. Is it possible to add click listener on the OK button so that I can do my stuff only when this OK button is pressed?</p>
  
  <p>Also is it possible to add specific ids to the OK and Cancel buttons so that I can distinguish them more easily, instad of using the pregenrated ids from ExtJS?</p>