GtkGradient: Handle symbolic gradients.

The css parser has been modified to parse correctly radial gradients:

background-image: -gtk-gradient (radial,
                                 center center, 0,
                                 center center, 0.8,
                                 from (#000), to (#fff));

The theming engine has been modified to correctly animate these,
as well as transitions between different pattern types.

gtk/gtksymboliccolor.h

@@ -49,6 +49,12 @@ GtkGradient * gtk_gradient_new_linear (gdouble x0,
                                        gdouble y0,
                                        gdouble x1,
                                        gdouble y1);
+GtkGradient * gtk_gradient_new_radial (gdouble x0,
+				       gdouble y0,
+				       gdouble radius0,
+				       gdouble x1,
+				       gdouble y1,
+				       gdouble radius1);
 
 void gtk_gradient_add_color_stop (GtkGradient      *gradient,
                                   gdouble           offset,