Drag & Drop on hidden elements

Determine if it would make sense to check if a droppable element is hidden. Suggested here:

https://forum.javascriptmvc.com/#Topic/32525000001014013

Demo:

http://jsfiddle.net/rgGmQ/1/

Start by reviewing the linked forum discussion and the jsFiddle demo to reproduce the hidden-element drag-and-drop behavior. Then locate the drag-and-drop entry point in jquerypp and define whether hidden droppable elements should be checked; done means the behavior is covered by an agreed result.

Written by the indexing model from the issue text.