The previous bottom navigation bottom bar was refactored, in order to be reused anywhere.
Then it was added to the product page.

Deleted files:
* `home_page.dart`
* `smooth_ui_library/smooth_bottom_navigation_bar.dart`
* `smooth_ui_library/smooth_bottom_navigation_bar_item.dart`

New file:
* `smooth_bottom_navigation_bar.dart`

Impacted files:
* `continuous_scan_page.dart`: added an explicit bottom navigation bar (was implicit through `HomePage`)
* `main.dart`: not using `HomePage` anymore; asking the bottom navigation bar who's the current page instead
* `new_product_page.dart`: added an explicit bottom navigation bar (was not there)
* `product_list_page.dart`: added an explicit bottom navigation bar (was implicit through `HomePage`)
* `product_page.dart`: added an explicit bottom navigation bar (was not there)
* `smooth_ui_library.dart`: removed "UI" version of smooth bottom navigation bar
* `user_preferences.dart`: added a preference for tab index
* `user_preferences_page.dart`: added an explicit bottom navigation bar (was implicit through `HomePage`)

Impacted files:
* `continuous_scan_page.dart`: explicitly select the "scan" tab
* `new_product_page.dart`: implicitly select the "default" tab
* `product_list_page.dart`: explicitly select the "history" tab
* `product_page.dart`: implicitly select the "default" tab
* `smooth_bottom_navigation_bar.dart`: now explicitly selecting the tab, instead of storing it in preferences
* `user_preferences.dart`: removed tab index preference
* `user_preferences_page.dart`: explicitly select the "profile" tab

Impacted file:
* `main.dart`: implicitly select the "default" tab and the matching page