The position child property is problematic, since it
requires us to emit notification for all children when
inserting a child early in the list of children.
Remove the property from all ui files.
The widget path code copies elements only in gtk_widget_path_copy() -
which is essentially unused - and in
gtk_widget_path_append_with_siblings() - which is used by GtkBox.
So stuff the widget we are testing in a GtkBox to reroduce the problem.
