rgba: Invert the arguments and improve bindability

Since parse() is a method of the Gdk.RGBA class, the GdkRGBA pointer
should be the first argument, and the string the second one, to allow a
more natural binding.

https://bugzilla.gnome.org/show_bug.cgi?id=635879

gtk/gtkbuilder.c

@@ -1563,7 +1563,7 @@ gtk_builder_value_from_string_type (GtkBuilder   *builder,
         {
           GdkRGBA rgba = { 0 };
 
-          if (gdk_rgba_parse (string, &rgba))
+          if (gdk_rgba_parse (&rgba, string))
             g_value_set_boxed (value, &rgba);
           else
             {