title: Animating "sliding" in NSOutlineView
body: <p>I've done a fair amount of Googling, but still haven't find any proper solution for animating the expansion of NSOutlineView.  I'm trying to mimic a "sliding" animation when expanding/collapsing rows.  Anyone know a good strategy for attempting this?</p>
  
  <p>Animating this:</p>
  
  <p>}} Folder 1</p>
  
  <p>To expand and look like this:</p>
  
  <p>}} Folder 1</p>
  
  <p>}}}}} Item 1</p>
  
  <p>}}}}} Item 2</p>
  
  <p>}}}}} Item 3</p>