title: make a UIBarButtonItem with customView behave like Flexible-Space item
body: <p>I have a <code>UIBarButtonItem</code> that has a <code>UITextField</code> as its customView.  The toolbar contains this item and a few buttons.</p>
  
  <p>As the toolbar resizes (say on a device-orientation change) I want the <code>UITextField</code> to grow or shrink (width) to occupy all the available space in the toolbar.</p>
  
  <p>Short of figuring out the combined sizes of all the buttons in the toolbar, then setting the <code>UITextField</code> width appropriately, is there just some magical way to make this item behave like a flexible-space bar item?</p>