Impacted files:
* `app_en.arb`: added a "remember my choice" and a "barcode already there" labels
* `app_fr.arb`: added a "remember my choice" and a "barcode already there" labels
* `price_amount_card.dart`: now using a focus node and a `key`; stronger resilience
* `price_amount_field.dart`: now using a focus node
* `price_amount_model.dart`: minor refactoring
* `price_model.dart`: new `getBarcodes` method
* `price_product_search_page.dart`: now managing a list of existing barcodes; now storing at session level the answer of the "server look-up?" question
* `product_price_add_page.dart`: now manages a focus on the latest added product, and keys for item deletion; not displaying the "add product" button if irrelevant