title: How to tell (programmatically) if there are / are not any registered apps that support opening a specific document type?
body: <p>Apple's documentation for UIDocumentInteractionController presentOpenInMenuFromBarButtonItem:animated: method states that "If there are no registered apps that support opening the document, the document interaction controller does not display a menu." In my app I want to display a button <strong>if and only if</strong> there is an app on the device that will open it. (I only want the button to pop up a menu to open a file; I don't want QuickLook, Copy or Print). As things stand, if the button is there, but no apps are registered that can open the relevant file, the user gets the unsatisfactory experience of a button that does nothing when tapped.</p>
  
  <p>So - can I find out whether or not there are any / no registered apps that support opening a specific document type? Clearly, UIDocumentInteractionController instances can find this out. Is there a public API way of finding it out?</p>