Add .clone method

Any Panel `Viewable` provides the `clone` method.

This makes it dead simple to make copies of instances for example for testing slight variations of the same object.

Please add. Thanks.


Start by locating how Panel Viewable objects integrate with this project and whether any existing copy or cloning behavior is exposed. Check the relevant API and test coverage before choosing an implementation point. Done means every Panel Viewable provides a clone method that makes an independent copy suitable for testing variations.

Written by the indexing model from the issue text.