Naturalize Drag and Drop

Instead of items being dropped to an index position in the list, they should be dropped "between" other items. This is a much more natural method of drag-and-drop, making it easier to organize download lists.

I don't know how the dropmarker location is controlled in this application, but it's a tiny bit off right now - depending on if you drop a selection in between two rows or on top of a row, the actual insertion location can be slightly different from what the dropmarker says.