Add class consistency_checker for domview_ui. - consistency_checker is to check consistency of the DOM-tree associated with a domview_ui. - It is so closed to domview_ui, so it may access private variables. - But, it uses public interface of domview_ui if possible. - consistency_checker is not integrated to domview_ui for separating functions of consistency checking from the domview_ui. It collects relative logic into a place and setups a boundary from others.