feat(input): drag and drop

[eladzlot]

This needs some more planning
maybe use this? http://www.quirksmode.org/js/dragdrop.html

[eladzlot]

from @baranan
One example for a task with drag&drop is an IAT in which participants drag the target stimulus to its category. However, perhaps that task is usually implemented with simple clicking on the categories, rather than dragging.
Another example is a task in which in each trial the user moves an image (e.g., snakes and puppies) to a manikin that represents him/her.

It seems fine to me to use an enableDrag attribute in media or stimuli. And a mouse event for drop with two handles for the stimuli sounds useful.

Most uses I have seen so far for drag and drop in studies is for ranking items (order by importance). But that might be better implemented as a question type in piQuest.