* helm.el (helm-define-key-with-subkeys): New macro.
* helm-files.el: Use it to bind new function helm-ff-delete-char-backward.
(helm-ff-delete-char-backward,
helm-ff-delete-char-backward--exit-fn): new.
* helm.el (helm-define-key-with-subkeys): Don't run funcall on nil if exit-fn is not specified.