(woman-mode-map): Copy button-buffer-map instead of

(woman-mode-map): Copy button-buffer-map instead of
making a new keymap.  Don't bind mouse-2.  Bind M-mouse-2 to
`woman-follow-word' instead of `woman-mouse-2'.
(woman-follow-word): Renamed from `woman-mouse-2'.
Follow unconditionally, since this function is now only
bound to M-mouse-2.  Use accessor functions.
(WoMan-highlight-references): Use `make-text-button'.
(woman-xref): New button type.