title: How do I get the width of the back button in a UINavigationBar?
body: <p>We have a controller that might be pushed from several places, so its back button might have one of several titles.  It also has a button that pops up another controller.  They both have the same custom title element, but they're centred differently because the second controller has no back button.  I'd like to add a spacer to fix this, but I don't know what width to make it.  How do I get the width of the back button in a UINavigationBar?</p>