[IMP]Export Wizard: Added Button to Sort fields in Fields_to_export

Hello,
I have improve by adding functionality to sort fields in Fields_to_export column in Export wizard

More Details:
Task: https://www.odoo.com/web#id=8723&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.f9cWyjK4y90DcjTl

thanks
rba-odoo