Add ability to return animate different thumbnail

This pull request adds the following functionality:

1. Once you swipe to reveal a new photo in the photo browser and you close the browser while a new photo is displayed, the new photo can be animated to its position on the underlying view (rather than just being dismissed downward off screen).
2. When you swipe to reveal a new photo in the browser, a callback can notify the underlying view of the change, so that it might scroll to the position of the photo being viewed in the browser.

I've updated the demo project accordingly.