[IMP] point_of_sale: UoS (Unit of Sale) Support.

 - UoS will appear on the Order, Receipts, Invoice as the unit of sale
 - The List Price will automatically be converted and rounded to the
   UoS based on the UoS Coeff.
 - The Order line quantity edition will attempt to respect the UoM
   rounding as well as the UoS rounding, in order to generate valid
   pickings
 - The product list still displays the price based on the UoM. Fixing
   this needs a bigger refactor, which is better left for the next
   release.