[FIX] export: allow excluding special virtual fields from exportable data
[FIX] export: allow excluding special virtual fields from exportable data

For example the virtual "Access Rights" fields on
res.users cannot be browsed and cannot currently
by exported.

havt/openerp-web,havt/openerp-web,havt/openerp-web,akretion/openerp-web,havt/openerp-web,havt/openerp-web,akretion/openerp-web,akretion/openerp-web