(custom-buffer-create-internal): Use widget type

(custom-buffer-create-internal): Use widget type
`custom-manual' instead of `info-link' and don't set properties
`button-face' and `mouse-face' explicitly.
(custom-browse-group-tag, custom-browse-variable-tag)
(custom-browse-face-tag): Inherit from widget class
`custom-group-link' instead of `push-button'.
(custom-button-unraised): Inherit from `underline' face.
(custom-link): Inherit from `link' face.  Fix doc and group.
(custom-add-parent-links, custom-group-value-create): Don't set
properties `button-face' and `mouse-face' explicitly for
`custom-group-link' widget.
(custom-group-link): Add properties `button-face' and `mouse-face'
to widget definition.
(custom-field-keymap): New variable.  Put `custom-field-keymap' to
editable-field's :keymap property.
(custom-mode): Fix docstring: substitute keybindings for
`widget-forward' and `widget-backward' from `widget-keymap',
`widget-complete' from `custom-field-keymap', replace old
`Custom-move-and-invoke' with `widget-move-and-invoke'.  Untabify.