Blender Git Loki

Stacked fullscreen area support (proper implemenation)

Adds support for stacked fullscreens. This basically means, if a user opens a
temporary fullscreen mode, such as the File Browser or the Image Editor render
view, from a different fullscreen, the "Back to Previous" function or the other
ways to escape those temporary fullscreens don't return to the split screen
layout but to the previous fullscreen he has been in.

I already committed something similar (f7e844570fea862) but that was only
supposed as a fix, it didn't work for the "Back to Previous" operator and the
implementation wasn't really reusable. This one looks a bit nicer + makes some
older hacks unnecessary :)