Generate a standard prefixed classname for the Widget, prefixed by the default prefix defined
by the 
Y.config.classNamePrefix attribute used by 
ClassNameManager and 
Widget.NAME.toLowerCase() (e.g. "yui-widget-xxxxx-yyyyy", based on default values for 
the prefix and widget class name).
The instance based version of this method can be used to generate standard prefixed classnames,
based on the instances NAME, as opposed to Widget.NAME. This method should be used when you
need to use a constant class name across different types instances.