easy-menu-define): Use ` and , read-macros

easy-menu-define): Use ` and , read-macros
instead of (` and (,.  Implement :filter.  Doc fix.
(easy-menu-do-define): Call `easy-menu-create-menu' instead of
`easy-menu-create-keymaps'.
(easy-menu-create-keymaps): Replaced by `easy-menu-create-menu'.
(easy-menu-create-menu): New public function.  Replaces
`easy-menu-create-keymaps', but with large changes.
(easy-menu-button-prefix): New constant.
(easy-menu-do-add-item, easy-menu-make-symbol): New functions.
(easy-menu-update-button): Doc fix.
(easy-menu-change): New optional argument BEFORE.
Now just a call to `easy-menu-add-item'.
(easy-menu-add-item, easy-menu-item-present-p)
(easy-menu-remove-item): New public functions.
(easy-menu-get-map, easy-menu-is-button-p, easy-menu-have-button-p)
(easy-menu-real-binding, easy-menu-change-prefix, easy-menu-filter):
New functions.