[IMP] purchase: usability improvements

- Task: https://www.odoo.com/web#id=5342&view_type=form&model=project.task&action=333&active_id=131
Overkill for me. When typing something in the search view, some PO / products / partners will be proposed, based on the input. I don't think we have to do something different than for other odoo models. I will remove it while merging.