title: How do I change/modify the displayed title of an NSPopUpButton
body: <p>I would like an <code>NSPopUpButton</code> to display a different title than the title of the menu item that is selected.</p>
  
  <p>Suppose I have an <code>NSPopUpButton</code> that lets the user pick a list of currencies, how can I have the collapsed/closed button show only the currencies abbreviation instead of the menu title of the selected currency (which is the full name of the currency)?</p>
  
  <p>I imagine I can override draw in a subclass (of <code>NSPopUpButtonCell</code>) and draw the entire button myself, but I would prefer a more lightweight approach for now that reuses the system's appearance.</p>
  
  <p>The menu items have the necessary information about the abbreviations, so that's not part of the question.</p>