Add a "Return To Default Columns" button

To support https://gitlab.developers.cam.ac.uk/uis/devops/uga/smi/-/issues/346 add a "Return To Default Columns" button to the dialog title. All it does is emit a suitably named event eg. onReset. The button should only display if a handler is provided for onReset.

Allow content of button to be customised.

Ensure that the stylguidist example illustrates how it is used.