Allow only initial selection for Nav

#### Is your feature request related to a problem? Please describe

A `tour.py` could improve its UX by preselecting nav item based on active hash.

#### Describe the solution you'd like

Introduce `initial_value` attr on `ui.nav_card`.

#### Describe alternatives you've considered

Tried to use `value`, but this allows app developers to manually control which nav item is currently active (clicking on other items does not work out of the box unless handled in python code via `value`). However, I only care about the initial selection without further overhead.