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.c

@@ -1356,6 +1356,10 @@ ensure_background (StThemeNode *node)
             {
               node->background_gradient_type = ST_GRADIENT_HORIZONTAL;
             }
+          else if (strcmp (term->content.str->stryng->str, "radial") == 0)
+            {
+              node->background_gradient_type = ST_GRADIENT_RADIAL;
+            }
           else
             {
               g_warning ("Unrecognized background-gradient-direction \"%s\"",