Point of Sale - New barcode rule for priced product with change rate

This module extends the Point of Sale barcode system so cashiers can scan barcodes that contain a price, and have that price automatically converted using a change rate before the product is added to the order.

• A new barcode rule type called "Priced Product with Change Rate" is available when configuring barcode rules (model barcode.rule).
• A new field "Change rate for barcode nomenclature" is added to the Point of Sale configuration (model pos.config) and to the general settings screen (model res.config.settings), shown inside the barcode scanner settings block.
• In the Point of Sale product screen, scanning a barcode that matches this rule looks up the product, then adds a line to the current order with the price from the barcode divided by the configured change rate.
• If no product is found for the scanned barcode, a "not found" notification is shown, just like the standard priced product rule.
• The module only affects the Point of Sale barcode reading flow and the related configuration screens; it does not change sales orders, inventory, or accounting.