Merging WEMI except for the XPM button implementation.

Merging WEMI except for the XPM button implementation.

* mime-view.el (widget-keymap): Declare.
(mime-view-maybe-inherit-widget-keymap): New function.
(mime-view-define-keymap-hook): Add
`mime-view-maybe-inherit-widget-keymap'.

* semi-def.el: Add setting for `widget-convert-button' to
autoload "wid-edit".
(mime-button-mother-dispatcher): Abolish.
(mime-button-dispatcher): Abolish.
(mime-add-button): Abolish.
(mime-button-action): New function.
(mime-button): New widget.
(mime-add-url-buttons): Rewrite with `url-link' widget.
(mime-button-face): Abolish.
(mime-button-mouse-face): Abolish.