Bug Fix in init

The lightbox gets created multiple times, as well as the navigation
list. So I had to add an nslider.loaded (bool) property, which is set
to true once the lightbox has been created. This is to prevent the
lightbox-making function from being called again for whatever reason.

Also made a minor adjustment to how nslider.thumb_nav_step is
generated. Just added an arbitrary 4 pixels to the generated amount.