title: How to set max width of a UIBarButtonItem
body: <p>I have a UIBarButtonItem in a UIToolbar that is updated with changes to a title represented by a text field.  The text field should not have a short maximum length.  When the title is quite long, the item occludes the button items to its right.  How can I make it automatically truncate to a certain width?
  <img src="https://i.stack.imgur.com/PbSiK.png" alt="Interface Builder">
  <img src="https://i.stack.imgur.com/FIgGX.png" alt="Simulator"></p>