title: How do you insert separators in something like an NSTableView?
body: <p>I'd like to learn how to use some kind of collection separator, such as in this example from Interface Builder:</p>
  
  <p><img src="https://i.stack.imgur.com/2msWr.png" alt="Interface Builder Example"></p>
  
  <p>I haven't seen anything about these online, but I don't think I'm searching with the right keywords. Can someone point me in a direction? </p>