New file:
* `background_task_queue.dart`: Queues for Background Tasks.
Impacted files:
* `background_task.dart`: now we add tasks to a specific queue
* `background_task_add_other_price.dart`: assigned to `BackgroundTaskQueue.fast`
* `background_task_add_price.dart`: assigned to `BackgroundTaskQueue.slow`
* `background_task_badge.dart`: minor refactoring
* `background_task_crop.dart`: assigned to `BackgroundTaskQueue.fast`
* `background_task_details.dart`: assigned to `BackgroundTaskQueue.fast`
* `background_task_download_products.dart`: assigned to `BackgroundTaskQueue.longHaul`
* `background_task_full_refresh.dart`: assigned to `BackgroundTaskQueue.longHaul`
* `background_task_hunger_games.dart`: assigned to `BackgroundTaskQueue.fast`
* `background_task_image.dart`: assigned to `BackgroundTaskQueue.slow`
* `background_task_language_refresh.dart`: assigned to `BackgroundTaskQueue.longHaul`
* `background_task_manager.dart`: now using new class `BackgroundTaskQueue` in order to use multiple queues
* `background_task_offline.dart`: assigned to `BackgroundTaskQueue.longHaul`
* `background_task_refresh_later.dart`: assigned to `BackgroundTaskQueue.fast`
* `background_task_top_barcodes.dart`: assigned to `BackgroundTaskQueue.longHaul`
* `background_task_unselect.dart`: assigned to `BackgroundTaskQueue.fast`
* `dao_string_list.dart`: added 2 queues
* `local_database.dart`: now running all queues
* `local_database_mock.dart`: minor refactoring
* `offline_tasks_page.dart`: now taking queues into account
* `up_to_date_mixin.dart`: now running all queues
* `user_preferences_page.dart`: now running all queues
