styleproperties: Add an unset vfunc to style properties

This makes unsetting work for shorthands.

gtk/gtkstyleproperties.c

@@ -319,6 +319,7 @@ gtk_style_properties_register_property (GtkStylePropertyParser  parse_func,
                                 NULL,
                                 NULL,
                                 NULL,
+                                NULL,
                                 NULL);
 }
 
@@ -839,6 +840,12 @@ gtk_style_properties_unset_property (GtkStyleProperties *props,
       return;
     }
 
+  if (node->unset_func)
+    {
+      node->unset_func (props, state);
+      return;
+    }
+
   priv = props->priv;
   prop = g_hash_table_lookup (priv->properties, node);