Use new menu item format. Don't simulate button prefix.

Use new menu item format.  Don't simulate button prefix.
(easy-menu-create-menu): Understand also keywords :active,
:label and :visible.  Don't worry about button prefix.
(easy-menu-button-prefix): Modified value.
(easy-menu-do-add-item): Extensive changes to use new menu item format.
(easy-menu-define-key, easy-menu-always-true): New functions.
(easy-menu-make-symbol): Don't use indirection for symbols.
Property `menu-alias' not set.
(easy-menu-filter, easy-menu-update-button): Deleted.
(easy-menu-add-item): Don't worry about button prefix.
(easy-menu-remove-item): Don't worry about button prefix.
Use `easy-menu-define-key'.
(easy-menu-is-button, easy-menu-have-button): Deleted.
(easy-menu-real-binding, easy-menu-change-prefix): Deleted.