title: Panning a UIView only from 1 subview
body: <p>I have a UIView (alertView) that serves as a custom alert. Inside this UIView, I have several pieces of interaction. I would like just the top portion of this custom alert (alertHeaderView) to be able to be panned to move the alertView, while still maintaining the functionality of the alertView contents.</p>
  
  <p>I currently have this half-working. In storyboard, I added the pan gesture recognizer to the whole alertView. My pan functionality works perfect, but it's preventing all the interaction from happening within the view. This is the problem.</p>
  
  <pre><code>@IBAction func panAlert(_ sender: UIPanGestureRecognizer) {
      // ... All my pan code.
  }
  </code></pre>
  
  <p>I've created an outlet for the header:</p>
  
  <pre><code>@IBOutlet weak var alertHeaderView: UIView!
  </code></pre>
  
  <p>Is there a way I can pan the entire alertView, ONLY from touches from alertHeaderView?</p>