Price-adding improvements

[monsieurtanuki]

Some improvements are needed so that the "add price" experience gets smoother:

• [x] feature: when we add a product, focusing on its "amount" text field
• [x] feature: not displaying the "add product" button when the product list only has the empty product
• [x] feature: not accepting a barcode that is already in the current product list
• [ ] #5417
• [x] feature: add a "remember my choice during this session" for the "server look-up?"dialog after each price product scan - good for online and offline contexts
• [x] bug: when we remove a product, there's some confusion with the amounts (looks like a typical flutter key issue)
• [ ] #5418

Part of

• #5203