Theme: Radial gradient background and enum for gradient type

This commit 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.

Reviewed By: billreynish, fclem, brecht

Differential Revision: https://developer.blender.org/D6825

source/blender/editors/include/UI_resources.h

@@ -309,7 +309,7 @@ typedef enum ThemeColorID {
   TH_GIZMO_A,
   TH_GIZMO_B,
 
-  TH_SHOW_BACK_GRAD,
+  TH_BACKGROUND_TYPE,
 
   TH_INFO_SELECTED,
   TH_INFO_SELECTED_TEXT,