This patch replaces the "Use Gradient" checkbox theme option with an
enum and implements a radial background.
Whith this change, it should be easier to implemet other types of more
complex background types, like a world space oriented gradient.
This was a tweak I made in a custom build when I was working on the
sculpt branch and I used it to record some demo videos, but it was
highly requested to be included in master.
The patch works, but I'm not sure on how to implement the versioning and
the compatibility with previously saved themes.