title: initWithBarButtonItem change from UIBarButtonSystemItemPause to UIBarButtonSystemItemPlay?
body: <p>My application has a <code>UIBarButtonItem</code> with a <code>UIBarButtonItemPause</code> icon in the upper right corner of the screen on a <code>UINavigationBar</code>. I'm looking to have it so that when I press the button it changes the icon to a play button, and back to a pause button when it is unpaused (similar to the way in which iTunes, Quicktime, or iPhone's integrated media player does when pressing the play/pause button. Is it possible for me to just change the icon or would it be necessary to create a new button each time and place it on the bar for that to happen? The code that is triggered by the pressing of the button can be found here:</p>
  
  <pre><code>- (IBAction)pauseapp:(UIBarButtonItem *)sender
  {
   if(paused==TRUE)
   {
    paused=FALSE;
    [pause initWithBarButtonSystemItem:(UIBarButtonSystemItemPause) target:self action:NULL];
    return;
   }
   else if(paused==FALSE)
   {
    paused=TRUE;
    [pause initWithBarButtonSystemItem:(UIBarButtonSystemItemPlay) target:self action:NULL];
    return;
   }
  </code></pre>