activate isearch in cascade menu

At present, isearch is not active automatically in the child menu even if do `popup-cascade-menu :isearch t`.
Because, the isearch condition is not passed on to `popup-cascade-menu`, which is called by `popup-select`/`popup-open` in `popup-menu-event-loop`.
But, I think it's not a natural behavior.

In addition, the parent menu is not focused by `popup-close` in the child menu.
So, I added the case of pushing the key that is equal to `popup-close` in `popup-isearch`.

Best regards.