… timestamps

History and Scan lists now behave differently from the other lists.
They use "extra data": a product is in the History (or the Scan) list
if it has as an "extra data" the "last seen timestamp" (or the last scan timestamp).

Bad news: as it's a new concept in the app, we'll start from scratch for both lists.

Impacted files:
* `continuous_scan_model.dart`: new way of adding a product to scanned products; refactoring
* `continuous_scan_page.dart`: unrelated color fix
* `dao_product.dart`: added "last seen" and "last scan" as extra data, with a direct access to products that have those extra data
* `dao_product_list.dart`: history and scan lists are now handled as "different" lists, computed from the "last seen" and "last scan" extra data; refactored
* `product_list.dart`: added extra `int` data; cleaner "same product list" test
* `product_list_dialog_helper.dart`: cleaner "same product list" test
* `product_list_page.dart`: added dates to the display
* `product_page.dart`: simplified history management
* `smooth_product_card_found.dart`: added an optional `refresh` parameter, after visiting the product page

…y and scan timestamps

In table `product_extra` we added an `int` column:
* the primary keys are barcode and "extra key" (e.g. an id for each different list)
* there is an `int` value and a `String` value
* the `int` value is used for ordering data (timestamps or 1,2,3,...)
* the `String` value is used for storing more complex data as json (e.g. the list of timestamps for history)

The contents of table `product_extra` can be stored in flutter code with the new class `ProductExtra` (basically, `int` and `String`)

For the moment the history and scan timestamps of each product are available via a lousy button at the end of the page.

New file:
* `product_extra.dart`: Extra data attached to a Product when it belongs to a ProductList

Impacted files:
* `dao_product.dart`: added an `int` column to the product extra table; now we use new class `ProductExtra`; refactored
* `dao_product_list.dart`: now we use new class `ProductExtra`; refactored
* `local_database.dart`: upgraded the db version because of a new column; added helper method `timestampToDateTime`
* `product_list.dart`: now we use new class `ProductExtra`; refactored
* `product_list_page.dart`: now we use new class `ProductExtra`
* `product_page.dart`: added a sample button at the end of the page in order to display all history and scans related to the product

…refresh timestamps

New files:
* `abstract_dao.dart`: DAO abstraction
* `dao_product_extra.dart`: DAO for Product Extra data

Impacted files:
* `barcode_product_query.dart`: automatically storing the product in the database
* `continuous_scan_model.dart`: refactored
* `dao_product.dart`: now extends new class `AbstractDao`; moved code to new class `DaoProductExtra`; refactored; optimized bulk actions
* `dao_product_list.dart`: now extends new class `AbstractDao`; now using new class `DaoProductExtra`; removed a SQLite foreign key for performance reasons; not storing products anymore (it's done somewhere else)
* `database_product_list_supplier.dart`: refactored because of `ProductListSupplier`
* `local_database.dart`: new database version; added `DaoProductExtra`
* `main.dart`: more verbose database crash message
* `product_dialog_helper.dart`: minor refactoring
* `product_list_supplier.dart`: refactored
* `product_page.dart`: now we display the access, the scan and the refresh timestamps in the lousy temporary button
* `product_query_model.dart`: removed the code regarding storing products in the database as it's now done in the `QueryProductListSupplier`
* `product_query_page.dart`: minor refactoring
* `product_query_page_helper.dart`: refactored because of `ProductListSupplier`
* `query_product_list_supplier.dart`: automatically storing the products in the database; refactored because of `ProductListSupplier`

…refresh timestamps

Impacted file:
* `continuous_scan_model.dart`: minor private variable renaming