title: Restrict NSTextField input to numeric only? NSNumberformatter
body: <p>I'm just getting started up with Mac App Development and so far everything is good well, I'm just having problems trying to get a NSTextField to only accept numbers for the input. </p>
  
  <p>So far I have added a formatter to the NSTextField and set it to only allow decimal now when I enter letters in the field it still allows me but does not allow my to click out of the cell when done. </p>
  
  <p>Ideally I would like the app not to let the user type in any letters and just beep or do nothing.</p>
  
  <p>Any tips or pointers on this would be great</p>