[ansel] Add next/back links to the screen view

[Brian Keifer]

Quoting Brian Keifer <brian at valinor.net>:

> Here's a patch to add 'next' and 'back' links to the screen view to navigate
> between images without having to go back to the gallery view.

...and there's a screenshot here:

http://valinor.net/ss/ansel-next-back-links.jpg

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