New file:
* `compare_products3_page.dart`: Test page about comparing 3 products. Work in progress.

Impacted files:
* `attributes_card_helper.dart`: refactored introducing two new reusable methods
* `nutrition_container.dart`: refactored introducing one reusable getter
* `product_cards_helper.dart`: refactored introducing one new reusable method
* `user_preferences_dev_mode.dart`: added an item computing 3 random products and showing the product comparison page

Impacted files:
* `product_list_page.dart`: slightly refactored the app bar text in selection mode, in order to have several possible actions; replaced the FAB with an explicit (?) "multi-select" button; added a "boosted comparison" icon button when 2 or 3 products are selected; added a "ranking" icon button
* `user_preferences_dev_mode.dart`: added a dev mode flag for "boosted comparison" (sic)

New file:
* `product_list_item_popup_items.dart`: Popup menu items for the product list page, for selected items.

Impacted files:
* `product_list_page.dart`: now displays item actions in a "more..." menu
* `product_list_popup_items.dart`: minor refactoring