[IMP] point_of_sale: allow a default category for the product list

 - This is useful when one point of sale sells a subset of the
   shop's products. The rest of the products are still available
   via the product list navigation bar.
[IMP] point_of_sale: allow different cashdrawer opening strategies.

 - Manual : only open when pressing the 'open cashbox' button
 - Automatic : open every time
 - Cash : only open when there is a cash payment
 - Disabled : never open the cashdrawer.
 - Any of the Manual / Automatic / Cash activates the pos proxy.