title: Extensions May not contain Stored properties
body: <p>Can I implement this in Swift with Extensions without the need to inheritance?. 
  I get this error 
  Extensions May not contain Stored properties</p>
  
  <pre><code>extension UIButton
  {
      @IBInspectable var borderWidth : CGFloat
          {
          didSet{
              layer.borderWidth = borderWidth
          }
      }
  
  }
  </code></pre>