Make proxyfield's WidgetDelegatedMethod lazily retrieve proxyfied method's func_code, so that it's easier to pinpoint underlying method.
Use case: when a widget rendering method raises and the rendered field is a proxyfield, func_code cans be read *after* call raised.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@21956 20353a03-c40f-0410-a6d1-a30d3c3de9de