…asks

New file:
* `work_type.dart`: Type of long download work for some background tasks.

Impacted files:
* `background_task.dart`: removed the default `uriProductHelper` getter
* `background_task_barcode.dart`: added a `uriProductHelper` getter that depends on the `productType`
* `background_task_download_products.dart`: refactored the access to product type
* `background_task_full_refresh.dart`: split by product type
* `background_task_language_refresh.dart`: split by product type
* `background_task_offline.dart`: refactored the access to product type
* `background_task_progressing.dart`: added the `productType` parameter; moved code to new `WorkType` class
* `background_task_top_barcodes.dart`: refactored the access to product type
* `dao_product.dart`: new methods `getProductTypes` and `splitAllProducts`; refactored with product type
* `lazy_counter.dart`: explicitly counting the "food" products
* `newsfeed_provider.dart`: explicitly getting the news from "food"
* `offline_data_page.dart`: now displaying "download top N products" buttons for each product type; stats for each product type
* `offline_tasks_page.dart`: enhanced "work text" algo, now depending on product type
* `operation_type.dart`: enhanced "key" algo, now depending on product type
* `ordered_nutrients_cache.dart`: explicitly using the "food" nutrients
* `product_list_page.dart`: now reloading products from their server
* `product_list_popup_items.dart`: now linking to the first server with products
* `product_query.dart`: made product type a mandatory parameter
* `product_refresher.dart`: added mandatory parameter product type
* `random_questions_query.dart`: explicitly ask for "food" robotoff products
* `temp_product_list_share_helper.dart`: added mandatory parameter product type
* `user_preferences_dev_debug_info.dart`: added explicit use of "food" data