Add dropdown selectors for navbar

[FlorianJacta]

Description

Add the possibility to have dropdown selectors for navbar and also think how pages can be organized to fit this case:

Acceptance Criteria

• If applicable, a new demo code is provided to show the new feature in action.
• Integration tests exhibiting how the functionality works are added.
• Any new code is covered by a unit tested.
• Check code coverage is at least 90%.
• Related issue(s) in taipy-doc are created for documentation and Release Notes are updated.

Code of Conduct

• I have checked the existing issues.
• I am willing to work on this issue (optional)