[IMP] point_of_sale: allow to choose loaded categs

Before this commit, it was not possible to effectively limit the available
products when a company had several POS with different product offers.

This commit allows limiting the available product categories for a single POS.

Now, a company can serve different POS with different product offerings.

Anecdotal Note: This was especially annoying as the POS only shows 100 products.
If those happened to be the products not served by a signle POS, the only work
around was using the search bar (on a tablet?!?)

closes odoo/odoo#28393