Preload prev/next images

When the lightbox is opened, or prev/next pressed, preloads the
previous/next N images.

Technical debt introduced:
* initialization is a mess, with the viewer and the interface
  randomly setting properties on each other in different phases of
  execution. That got in the way and I shuffled things around
  until they worked, which is obviously not the way to have a
  robust system, but hopefully it will get scrapped soon anyway
  in favor of a clean top-down dependency injection.

Change-Id: Idcb5c40de1ac0b3e482decd66e56c4de8ec71b6b
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/155