Impacted files:
* `get_prices_model.dart`: added a `LazyCounterPrices?` parameter
* `lazy_counter.dart`: added a `bool notify` parameter
* `lazy_counter_widget.dart`: now refreshed by "external" new count
* `price_user_button.dart`: added a `LazyCounterPrices` parameter to have the counter refreshed
* `product_prices_list.dart`: now setting the lazy counter when displaying the list
* `user_preferences.dart`: added a `bool notify` parameter for the lazy counter setter
* `user_preferences_account.dart`: added a `LazyCounterPrices` parameter to have the counter refreshed
New files:
* `lazy_counter.dart`: Lazy Counter, with a cached value stored locally, and a call to the server.
* `lazy_counter_widget.dart`: Widget displaying a Lazy Counter: cached value, refresh button, and loading.
Impacted files:
* `user_preferences.dart`: added methods about storing a lazy count value
* `user_preferences_account.dart`: refactored the display of counts with the new `LazyCounter` and `LazyCounterWidget` classes