title: Callback to sectionHeadersPinToVisibleBounds
body: <p>I have a requirement in my project that I have to add shadow to UICollectionView headers when they become sticky. </p>
  
  <p>I am using the property on UICollectionViewFlowLayout sectionHeadersPinToVisibleBounds, which when set to YES makes the headers sticky.</p>
  
  <p>Can we get some sort of callback when the headers become sticky and actually pin to top so that I can add a shadow to that header? I don't want the shadow to be there when a header is not sticky.</p>
  
  <p>The challenge here is, I can't really make use of the information in method- layoutAttributesForSupplementaryView: which does provide me the views on screen as my sections having dynamic no of items, hence which section is on screen doesn't really tell me who needs a shadow.</p>