(easy-menu-create-keymaps): Menu item STYLE toggle (checkbox)
and radio (radio button) are prefixed by "[X] " or
"(*) " respectively, when selected and "[ ] " or "( ) ", when not
selected. In a menu that contain these prefixes, " " is used
as prefix for items that have no other prefix.
(easy-menu-update-button): New function used as `menu-enable'
property for checkboxes and radio buttons to update the prefixes.
(easy-menu-define): Change documentation string to describe the
new prefixes.