(widgets) [defgroup]: Remove url link.

(widgets) [defgroup]: Remove url link.
(widget-color-choice-list, widget-color-history, widget-mouse-help):
Deleted.
(widget-specify-field, widget-specify-button): Don't use
widget-mouse-help as help-echo property.
(default): Use #'ignore for :validate and :mouse-down-action.
(checkbox): Add help-echo.
(widget-sexp-validate): Rewritten to clarify error messages.
(character): Use char-valid-p in :match function.
(widget-color-complete): Use facemenu-color-alist.
(widget-color-action): Use facemenu-read-color.