title: NSCollectionView with sections - like in iPhoto
body: <p>I'd like to build a NSCollectionView similar to the one in iPhoto '11. I want to group several pictures in section, and create a section header as well. The section header of a specific section is always visible until the last element of that section is visible. You can take a look at the picture to see what I mean.</p>
  
  <p>EDIT: I should add that the contents are not images.</p>
  
  <p><img src="https://i.stack.imgur.com/Ub32n.png" alt="enter image description here"></p>
  
  <p>Thanks in advance</p>