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

[brechtvanlommel at gmail.com]

Steps to redo with latest patch:
* Load factory settings
* Change top info space type to another type
* Drag up separator between 3d view and this space to collapse it, and
release
* Afterwards, the space is not collapsed and the outliner header on the
right is partially hidden until it refreshes when you mouse over the
outliner.

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