Use a buffer to show marked candidates for action.
* helm-utils.el (with-helm-display-marked-candidates): New macro.
* helm-files.el (helm-find-files-do-action,
                 helm-delete-marked-files): Use it.
* helm-files.el (helm-find-files-do-action): Keep position when quitting.
Take advantage of dired-format-columns-of-files.
* helm-files.el (helm-find-files-do-action,
                 helm-delete-marked-files): Do it.