Make hiding of scrollbars configurable

[arnowelzel]

Also see https://wordpress.org/support/topic/disable-hiding-scrollbars

One user reported, that hiding the scrollbars makes opening the lightbox less "smooth" when using the Enfold theme in WordPress and asks to be able to disable this feature.

This would be an additional option in the backend settings which controls if hideScrollbar() and showScrollbar() in the frontend will be used when opening the lightbox.

[arnowelzel]

Closed with a43aeb0