[IMP] purchase: misc usability improvements
- RFQ wording imp
- products tree view in purchase form view: set account_analytic_id
for group_analytic_accounting only + add a context for the partner
- purchase: res_config: wording imp + config view imp
[IMP] web: order fields in export wizard

(Closes #1645)
[IMP]Export Wizard: Added Button to Sort fields in Fields_to_export column