[accounting] New API migration - WORK IN PROGRESS

- Task: https://www.odoo.com/web/#id=9955&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.Uy4wEA7I4G4GwPaU
convert also the overwriting of the onchange method to new API in inherited modules: this will not increase the diff, it will reduce it because currently you have to define twice the onchange method, and _that_'s making the diff bigger.