title: tvOS UICollectionView get currently focused item's index path
body: <p>So this is kind of a noobish question but I just can't figure out a very simple way to detect currently focused item's <code>indexPath</code>.<br>
  I looked around hoping to see something very easy like <code>collectionView.indexPathOfCurrentlyFocusedItem</code> but didn't find anything remotely close.
  So I digged around and tried to find something similar at <code>UIFocusEnvironment</code>, <code>UIFocusUpdateContext</code> trying to find the desired property but failed.
  So, the only solution I can come up with is just iterating through all visible cells and finding a cell with focused property set to true.</p>
  
  <p>So is there a more simple and elegant way to find the currently focused item's <code>indexPath</code>? <strong>(Except tracking it through delegate method and saving it in view controller's property)</strong></p>