Implement radial gradients for StWidget

Some theme authors have stated interest in radial gradient backgrounds.
The w3c has some draft:

http://dev.w3.org/csswg/css3-images/#radial-gradients

As this is rather complex, we add only some very basic support, which
extends our syntax for linear gradients:

background-gradient-direction: [vertical|horizontal|radial]

Gradients are centered circles, whose size is determined by the closest
side.

https://bugzilla.gnome.org/show_bug.cgi?id=604945

src/st/st-theme-node.h

@@ -61,7 +61,8 @@ typedef enum {
 typedef enum {
   ST_GRADIENT_NONE,
   ST_GRADIENT_VERTICAL,
-  ST_GRADIENT_HORIZONTAL
+  ST_GRADIENT_HORIZONTAL,
+  ST_GRADIENT_RADIAL
 } StGradientType;
 
 GType st_theme_node_get_type (void) G_GNUC_CONST;