title: iOS UICollectionView cells' transition/slide in animation when they are about to appear
body: <p>What I already have: a horizontally scrolled <code>UICollectionView</code>, and its cells (just a single line of cells).</p>
  
  <p>What I want to do: animate the cells when they are about to appear -- <code>cells</code> slide in from right (outside the screen) to left, eventually stop at the left border of the <code>UICollectionView</code>.</p>
  
  <p>How could I achieve this? Any help is greatly appreciated!</p>
  
  <p><img src="https://i.stack.imgur.com/6Hv4R.png" alt="Question explained in this picture..."></p>