Support list items of variable height

If a list is composed of items of varying height, when dragging a smaller item over a large item, one will see flickering as the placeholder is placed before the item, then after the item, and back again, infinitely.

jQuery-UI uses something approximating a dead zone, where the swap will only occur over an area equal to the item being dragged. For example, if dragging a 20px tall item over a 40px tall item, no swap will occur over the first 20px while dragging from above, or over the last 20px while dragging from below.

This small patch creates a deadzone inside larger items, resolving the flicking issue.