feat: 5403 - improvements for "price adding"

[monsieurtanuki]

What

Improvements about price adding:

• when we add a product, now we focus on its "paid price" text field
• now we don't display the "add product" button when the product list only has the empty product
• we don't accept twice the same product
• now there's a "remember my choice during this session" checkbox regarding automatic server look-up after scan

Bug fix:

• confusion with the amounts when we remove a product (typical flutter key issue)

Screenshot

unique barcode	no "add product" button at start	remember my choice

Part of

• 5403

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

[monsieurtanuki]

Thank you @g123k for your comments, it made me go back to the flutter docs and realize that I forgot the dispose calls. I've just added them in the latest push.

As for your latest comments, I guess you've reached the limits of my understanding: I cannot picture in which language a number like 5 wouldn't be understood (OK Japanese people also use 五 but they do understand 5), and a TextEditingController won't notify an object that doesn't exist yet.

Feel free to amend this PR or to close it.

Meanwhile, I convert it to "Draft".

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 93 lines in your changes missing coverage. Please review.

Project coverage is 6.94%. Comparing base (4d9c7fc) to head (5fb06b8). Report is 284 commits behind head on develop.

Files	Patch %	Lines
...pp/lib/pages/prices/price_product_search_page.dart	0.00%	35 Missing :warning:
...smooth_app/lib/pages/prices/price_amount_card.dart	0.00%	28 Missing :warning:
...h_app/lib/pages/prices/product_price_add_page.dart	0.00%	22 Missing :warning:
...kages/smooth_app/lib/pages/prices/price_model.dart	0.00%	4 Missing :warning:
...mooth_app/lib/pages/prices/price_amount_model.dart	0.00%	3 Missing :warning:
...mooth_app/lib/pages/prices/price_amount_field.dart	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #5404 +/- ## ========================================== - Coverage 9.54% 6.94% -2.61% ========================================== Files 325 399 +74 Lines 16411 21203 +4792 ========================================== - Hits 1567 1473 -94 - Misses 14844 19730 +4886 ```

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[monsieurtanuki]

Closing as stale.