(help-mode-map): Make button-buffer-map our parent.

(help-mode-map): Make button-buffer-map our parent.
Don't bind mouse events or tab/backtab.
(help-function, help-variable, help-face, help-coding-system)
(help-input-method, help-character-set, help-back, help-info)
(help-customize-variable,  help-function-def, help-variable-def):
New button types.
(help-button-action): New function.
(describe-function-1): Pass help button-types to `help-xref-button'
rather than help function and help-echo string.  Don't put multiple
help-function args in a list to pass them to help-xref-button, just pass
them as multiple arguments.  Use `help-insert-xref-button' to make
[back]-button, rather than `help-xref-button'.
(help-xref-button): Take a button-type TYPE as a parameter rather than a
function.  Remove HELP-ECHO parameter.  Remove DATA parameter and add a
&rest parameter ARGS to serve the same purpose.  Use `make-text-button'
to add the button.
(help-insert-xref-button): Use `insert-text-button' to add the button.
(help-follow-mouse, help-next-ref, help-previous-ref): Functions removed.
(help-do-xref): New function.
(help-follow): Use `push-button' and `help-do-xref' to do most of the work.