Reorganized tabs

There was a bug with the original onchange implementation: When the client was viewing a subpage of the navbar and then resized the window the onchange also fired and thus redirected the user to the main "events" page for example.

This reorganization cleans the tabs code, BUT it removes the flexibility of the old tabs code. Now the tabs component can onlz be used for the navbar.

@lic What is your opinion on less flexible but cleaner code? We could also refactor the implementation into a different folder to make it clear that it is not a general component