Deleted file:
* `background_task_helper.dart`
New files:
* `background_task_manager.dart`: Management of background tasks: single thread, block, restart, display.
* `dao_instant_string.dart`: Where we store strings that need INSTANT access (= not lazy, no await).
Impacted fles:
* `abstract_background_task.dart`: refactored
* `background_task_details.dart`: refactored around the changes in `AbstractBackgroundTask`
* `background_task_image.dart`: refactored around the changes in `AbstractBackgroundTask`
* `dao_string_list.dart`: refactoring around now managing several lists; removed unnecessary `await` for a non-lazy dao
* `local_database.dart`: added the new class `DaoInstantString`; relaunch the background task manager at every refresh
* `main.dart`: minor refactoring
* `new_crop_page.dart`: unrelated bug fix
* `offline_tasks_page.dart`: refactored around the new `BackgroundTaskManager`
* `operation_type.dart`: added helper methods
* `product_image_gallery_view.dart`: minor refactoring
* `product_image_viewer.dart`: unrelated bug fix - the product was not refreshed, and so wasn't the image even after a successful download
* `pubspec.lock`: wtf
* `pubspec.yaml`: removed `flutter_task_manager`
* `search_history_view.dart`: minor refactoring now that we have several lists in `DaoStringList`
* `search_page.dart`: minor refactoring now that we have several lists in `DaoStringList`
* `up_to_date_changes.dart`: minor refactoring
* `up_to_date_product_provider.dart`: minor refactoring
