The way CheckBoxMenuImplementation keeps track of selected items is inappropriate

[lenormf]

Describe the bug
The CheckBoxMenuImplementation keeps track of the selected items using a map (_selected_item_dict), whose keys are the items themselves and assigned a boolean value (whether they're selected).

This causes several issues:

• all items added to the menu are unchecked by default, but added to the map nonetheless, which duplicates data unnecessarily
• distinct items with the same text will all share the same “selected” status as soon as one changes
• checking/unchecking all items requires a loop (to assign True/False to every entry)

Intuitively I'd say an array containing the “element identifier” (an integer incremented at every add, or a more elaborate hash) of all items marked as selected should be used instead.