Dragging a widget to another sidebar causes Add a Widget to erroneously move

[westonruter]

From Nacin in IRC:

funny bug: when you drag from one widget area to another, and that second area is currently empty, the space opens up below "Add a Widget", even though when dropped it'll appear above them.

[westonruter]

This would probably be resolved in #77 since the li containing the Add a Widget button would not be a sibling of the widget form control elements.