Implement gradients for StWidget

Rather than having gradients be individually implemented by higher
level JS widgets, move basic gradient functionality into StWidget.
There is prior art in WebKit for CSS gradients:

http://webkit.org/blog/175/introducing-css-gradients/

However, implementing this would be quite a lot of work; all we
need in the Shell design at the moment is basic horizontal/vertical
linear gradients.  So, the syntax now supported is:

background-gradient-type: [vertical|horizontal]
background-gradient-start: color;
background-gradient-end: color;

https://bugzilla.gnome.org/show_bug.cgi?id=602131

src/st/st-theme-node.c

@@ -21,8 +21,13 @@ struct _StThemeNode {
   PangoFontDescription *font_desc;
 
   ClutterColor background_color;
+  /* If gradient is set, then background_color is the gradient start */
+  StGradientType background_gradient_type;
+  ClutterColor background_gradient_end;
+
   ClutterColor foreground_color;
   ClutterColor border_color[4];
+
   double border_width[4];
   double border_radius[4];
   guint padding[4];
@@ -1236,6 +1241,7 @@ ensure_background (StThemeNode *node)
 
   node->background_computed = TRUE;
   node->background_color = TRANSPARENT_COLOR;
+  node->background_gradient_type = ST_GRADIENT_NONE;
 
   ensure_properties (node);
 
@@ -1331,6 +1337,31 @@ ensure_background (StThemeNode *node)
               node->background_image = NULL;
             }
         }
+      else if (strcmp (property_name, "-gradient-direction") == 0)
+        {
+          CRTerm *term = decl->value;
+          if (strcmp (term->content.str->stryng->str, "vertical") == 0)
+            {
+              node->background_gradient_type = ST_GRADIENT_VERTICAL;
+            }
+          else if (strcmp (term->content.str->stryng->str, "horizontal") == 0)
+            {
+              node->background_gradient_type = ST_GRADIENT_HORIZONTAL;
+            }
+          else
+            {
+              g_warning ("Unrecognized background-gradient-direction \"%s\"",
+                         term->content.str->stryng->str);
+            }
+        }
+      else if (strcmp (property_name, "-gradient-start") == 0)
+        {
+          get_color_from_term (node, decl->value, &node->background_color);
+        }
+      else if (strcmp (property_name, "-gradient-end") == 0)
+        {
+          get_color_from_term (node, decl->value, &node->background_gradient_end);
+        }
     }
 }
 
@@ -1393,6 +1424,34 @@ st_theme_node_get_foreground_color (StThemeNode  *node,
   *color = node->foreground_color;
 }
 
+
+/**
+ * st_theme_node_get_background_gradient:
+ * @node: A #StThemeNode
+ * @type: (out): Type of gradient
+ * @start: Color at start of gradient
+ * @end: Color at end of gradient
+ *
+ * The @start and @end arguments will only be set if @type is not #ST_GRADIENT_NONE.
+ */
+void
+st_theme_node_get_background_gradient (StThemeNode    *node,
+                                       StGradientType *type,
+                                       ClutterColor   *start,
+                                       ClutterColor   *end)
+{
+  g_return_if_fail (ST_IS_THEME_NODE (node));
+
+  ensure_background (node);
+
+  *type = node->background_gradient_type;
+  if (*type != ST_GRADIENT_NONE)
+    {
+      *start = node->background_color;
+      *end = node->background_gradient_end;
+    }
+}
+
 void
 st_theme_node_get_border_color (StThemeNode  *node,
                                 StSide        side,