title: NSArrayController : removeAllObjects does not refresh TableView
body: <p>In my application I add objects directly to an ArrayController. When I want to clean all items I do:</p>
  
  <pre><code>[[downloadItemsController content] removeAllObjects];
  </code></pre>
  
  <p>This command however does not refresh the TableView the arraycontroller is bound to. If I remove all items and add another new items I only see that item. That is fine but if I don't add anything I still have all my items in the table.</p>
  
  <p>If I do</p>
  
  <pre><code>[downloadItemsController prepareContent];
  </code></pre>
  
  <p>all old items are removed from the tableview but than I will get an new and empty item I can edit. I don't want that and because one of my columns has a checkboxcell I always get an row with a checkbox.</p>
  
  <p>I just need an empty table with no items after I remove all existing items.</p>