Add GtkGradient.

This type defines a gradient between 2 symbolic colors, with any number
of color stop between these. At the moment it has been put besides
GtkSymbolicColor, although should be completely private, it will likely
need extending in the future for radial gradients.

At some point, it maybe should also be used for images, so painting
both a gradient and an image is consistent.

gtk/gtksymboliccolor.h

@@ -26,11 +26,13 @@ G_BEGIN_DECLS
 
 /* Dummy typedefs */
 typedef struct GtkSymbolicColor GtkSymbolicColor;
-typedef struct GtkSymbolicColorMap GtkSymbolicColorMap;
+typedef struct GtkGradient GtkGradient;
 
 #define GTK_TYPE_SYMBOLIC_COLOR (gtk_symbolic_color_get_type ())
+#define GTK_TYPE_GRADIENT (gtk_gradient_get_type ())
 
 GType gtk_symbolic_color_get_type (void) G_GNUC_CONST;
+GType gtk_gradient_get_type (void) G_GNUC_CONST;
 
 GtkSymbolicColor * gtk_symbolic_color_new_literal (GdkColor         *color);
 GtkSymbolicColor * gtk_symbolic_color_new_name    (const gchar      *name);
@@ -43,6 +45,18 @@ GtkSymbolicColor * gtk_symbolic_color_new_mix     (GtkSymbolicColor *color1,
 GtkSymbolicColor * gtk_symbolic_color_ref   (GtkSymbolicColor *color);
 void               gtk_symbolic_color_unref (GtkSymbolicColor *color);
 
+GtkGradient * gtk_gradient_new_linear (gdouble x0,
+                                       gdouble y0,
+                                       gdouble x1,
+                                       gdouble y1);
+
+void gtk_gradient_add_color_stop (GtkGradient      *gradient,
+                                  gdouble           offset,
+                                  GtkSymbolicColor *color);
+
+GtkGradient * gtk_gradient_ref   (GtkGradient *gradient);
+void          gtk_gradient_unref (GtkGradient *gradient);
+
 G_END_DECLS
 
 #endif /* __GTK_SYMBOLIC_COLOR_H__ */