[IMP] mrp: usability of BoM (lines)

- Task: https://www.odoo.com/web#id=10010&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.FJILGyANBSVkazkw
Hmm, kits and phantom BoM are closely related in terms of semantics. Yes, the former is controlled at the BoM level and used for the Sales/Shipping process, and the latter at the line level, and used for the Manufacturing process, but both represent a "substitution" for a list of products & quantities.

Shouldn't we be more consistent in handling those 2 concepts? Why switch one to a boolean and keep the other as a selection? What about their visibility (usability), should they always be displayed at the same time? Do we need a UI setting for enabling them, although it may seem overkill indeed ...

@anajuaristi What do MRP experts have to say about this? :-)