* lisp/button.el: Make them work in header-lines.

* lisp/button.el: Make them work in header-lines.
(button-map): Add bindings for header-line and mode-line use.
(button-get, button-put, button-label): `button' may now be a string.
(button-activate): Don't make it a defsubst.
(button--area-button-p, button--area-button-string): New functions.
(make-text-button): Fix the return value when `beg' was a string.
(push-button): Handle the mode-line case.