Remove final from classes

Problem/Motivation

Currently almost all classes of the Navigation module are marked as internal and the classes are marked as final. Sometimes there is a comment like "The navigation module is experimental.".

I understand that some classes may be pure internal, but for plugins Top Bar items or blocks, it prevents people to override it into their projects if needing to adjust behavior.

I also understand that this module is still experimental but if people wants to extend it I would say it is their choice despite the experimental warnings. The code should not block them.

Proposed resolution

Is it already planned to have (some) final removed?

Remaining tasks

API changes

Release notes snippet