title: Text field on UIAlertController triggers disabled button
body: <p>I have a <code>UIAlertController</code> with a <code>UITextField</code> and a submit button. The code looks like this:</p>
  
  <pre><code>let saveDialogue = UIAlertController(title: "Save Level",
      message: "",
      preferredStyle: .Alert)
  
  let saveAction = UIAlertAction(title: "Save", style: .Default,
      handler: { (action) in
          let textfield = saveDialogue.textFields![0] as UITextField
          // do something with the textfield
      }
  )
  saveAction.enabled = false
  saveDialogue.addTextFieldWithConfigurationHandler { (textField) in
      textField.placeholder = "My Level"
      textField.text = filename
      // conditionally enable the button 
      textField.addTarget(self, action: "textChanged:", forControlEvents: .EditingChanged)
  
  }
  </code></pre>
  
  <p>While the button cannot be tapped, hitting the return key on the keyboard triggers the default action (<code>saveAction</code>).</p>
  
  <p>Is there a way around this? I also tried to validate the textfield value in the handler, but the view will be dismissed. Can it be retained?</p>