widgetpath: Use ref/unref in boxed function

This way we avoid lots of copying when using widget paths as properties.

Keeping this as a separate patch so bugs where people were modifying a
structure they shouldn't are easily visible.

gtk/gtkwidgetpath.c

@@ -82,7 +82,7 @@
  **/
 
 G_DEFINE_BOXED_TYPE (GtkWidgetPath, gtk_widget_path,
-		     gtk_widget_path_copy, gtk_widget_path_free)
+		     gtk_widget_path_ref, gtk_widget_path_unref)
 
 
 typedef struct GtkPathElement GtkPathElement;