title: UISplitViewController in portrait: how to hide master popover programmatically?
body: <p>In my <code>UISplitViewController</code> the master controller is a <code>UINavigationController</code>.
  When in portrait mode I would like to keep the navigation controller visible as long as the user navigates upwards (using the back button). As soon as an item from the table view of the navigation controller is selected, I want to dismiss the popover.
  How can I achieve this? How can my <code>UITableViewController</code> know if it is inside a popover, and if yes, dismiss itself?</p>