title: UITextView backed by NSTextStorage not updated when text appended
body: <p>The following is a trivial example of a UITextView backed by a NSTextStorage. The "doAction1" is fired by clicking on some UI button. Reading through the docs, I was under the impression that by updating the NSTextStorage in a begin/end Editing block, the View will automatically update itself as well. </p>
  
  <p>However, it doesn't seems to happen -- the example below appends "...World!" to NSTextStorage but the UITextView doesn't reflect that -- and continues to just display "Hello". Any idea?</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
    let TEXT = "Hello"
  
    var m_layoutManager: NSLayoutManager!
    var      m_textView: UITextView!
  
    @IBAction func doAction1(_ sender: AnyObject) {
        let ts = m_layoutManager.textStorage
  
        ts?.beginEditing()
        ts?.append(AttributedString(string:"...World!"))
        ts?.endEditing()
  
        // These two don't make a difference either:
        //
        // m_layoutManager.invalidateDisplay(forCharacterRange: NSRange(location: 0, length: (ts?.length)!))
        // m_textView.setNeedsDisplay()
        //
    }
  
    override func viewDidLoad() {
      super.viewDidLoad()
  
      let textStorage = NSTextStorage(string: TEXT)
      let textContainer = NSTextContainer(
          size: CGSize(width: 200, height: 300))
  
      m_layoutManager = NSLayoutManager()
      m_layoutManager.textStorage = textStorage
      m_layoutManager.addTextContainer(textContainer)
  
      m_textView = UITextView(
          frame: CGRect(x: 0, y: 20, width: 200, height: 300),
          textContainer: textContainer)
  
      view.addSubview(m_textView)
    }
  
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
  }
  </code></pre>