proxyfield, because the renderer class was using the target listbox field.
Handle the case of proxy field specificly so that the original proxyfield is
used by the renderer, and calls to get_value are looked up properly.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@15729 20353a03-c40f-0410-a6d1-a30d3c3de9de