Lightbox: fix mouse wheel behavior and add horizontal scroll feature.

[veroxzik]

This addresses my issue #835 and is two fold:

1. Previously, any non-negative deltaY change to the mouse wheel would cause a photo in the lightbox zoom out, regardless of the direction. I have changed this so that zoom out is only possible with positive deltaY values.
2. I have added the option to scroll through photos in the lightbox using horizontal scrolling (deltaX). This mimics the behavior of left/right arrow and allows for navigation solely with the mouse. I've also changed it so you can use this scroll on videos. The previous behavior of canceling zoom on videos remains.

Thanks for the consideration!