Eyecandy feature: background gradient for 3D viewport. Enable in user preferences under themes->3D view->Theme Gradient Color. This is only used when use render only is not ticked and for now it may interfere with grid lines. Will investigate how to adjust contrast.

Tidying up of options after advisory session on irc: Move all RNA code
in Themes.

Changes after merging trunk's commit that renders sky

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53608 954f8c5b-7b00-dc11-b283-0030488c597c