title: Change scrolling position of collection view, when view is currently not visible
body: <p>There are two scenarios where I have problem with setting the scrolling position of my collection view:</p>
  
  <ul>
  <li>on start up I want to scroll to a certain item (starting position != beginning of the collection view)</li>
  <li>view is currently not visible yet, but in "background" the collection view should scroll to a certain item</li>
  </ul>
  
  <p>What is a reliable way to scroll the collection view to a certain cell? Currently I'm setting the scrolling position with <code>scrollToItemAtIndexPath</code> in <code>viewDidAppear</code>, but that is too late for me.</p>