Accommodate changes to the lightbox HTML

A patch to accommodate the required changes, including:

• Always specifying a data-lightbox-type.
• Removed the HTML lightbox checkbox now that it is unnecessary.
• Identifying lightbox anchors with a static class instead of generated ID.
• Allowing users to manually specify a gallery ID for files outside of an image field.
• Added descriptions to file field formatter options.
• Added a function to retrieve available themes to reduce code duplication and improve performance.

Tested #1 and committed to Nivo Lightbox 7.x-1.x.

Automatically closed - issue fixed for 2 weeks with no activity.