* wl-xmas.el (wl-message-overload-functions): Initialize

* wl-xmas.el (wl-message-overload-functions): Initialize
`wl-message-button-map'.

* wl-mule.el (wl-message-overload-functions): Initialize
`wl-message-button-map'.

* wl-vars.el (wl-highlight-message-header-button-alist): New.

* wl-message.el (wl-message-button-map): New keymap.
(wl-message-add-button): New function.
(wl-message-button-dispatcher): New function.
(wl-message-button-refer-article): New function.

* wl-highlight.el
(wl-highlight-message-add-buttons-to-header): New function.
(wl-highlight-headers): Use it.