[Bf-codereview] Drag to close areas (issue 6269044)

[jorge at lunarworkshop.com]

I chose to do it the simple way for now. I've added the multiple areas
problem to my to do list and I'll handle it at a later time, when I can
focus on making it work for more than just this feature, but also the
right-click method and the drag-handle method. Until then, users can't
drag splits with multiple areas on either side closed, and the behavior
in that case remains the same as it was before.

http://codereview.appspot.com/6269044/