title: Populating NSPopupButton with Core Data Entity plus a menu Item
body: <p>Is there a way to put the list of a core data entity into a NSPopupButton and additionally put a menu item there?</p>
  
  <p>I have two core data entities, say Person and Car.
  A Person has many Cars (relationship).
  On a sheet the user should be able to pick a car from a popup button for the person.
  This popup button should also offer an option to create a new car, that would then be selected for that person.</p>
  
  <p>My attempts so far:</p>
  
  <p>The items I put into the NSPopupButton in IB do not appear at runtime when the popup is populated from core data.</p>
  
  <p>I can specify an action that gets called when the user selects an item from the popup, but that gets triggered for every item. (I needed to have a dummy entry in the core data entity only to trigger the desired sheet, also I cannot control the position of that dummy item in the popup)</p>