title: How is a NSPopUpButton filled with data when using Core Data and Bindings?
body: <p>I'm trying to figure out how a <code>NSPopUpButton</code> is filled with data when using core data and cocoa bindings. The reason is that I subclassed <code>NSPopUpButton</code> and I would like to manipulate the insertion of the menu items by adding an image to them.</p>
  
  <p>I subclassed 
  <code>- (void)addItemWithTitle:(NSString *)title</code>,
  <code>- (void)addItemsWithTitles:(NSArray *)itemTitles</code> and 
  <code>- (void)insertItemWithTitle:(NSString *)title atIndex:(NSInteger)index</code>
  just to see which method is called but none of them is on application start. </p>
  
  <p>Does anybody know what's the correct method . The class reference doc isn't very helpful here (or I didn't get it how to use it) as there are no other methods listed which affect the item insertion.</p>
  
  <p>Thanks in advance,
  Thomas</p>