Custom Handlers for Listers: 'unsnapshot'

The unsnapshot event will be sent to your handler when the user un-snapshots
a lister under your control.

Its arguments are:-

        Arg0 - "unsnapshot"        (a string indicating the event type)
        Arg1 - <handle>            (lister handle)

It is up to you to remember that the lister has been un-snapshotted if you
wish to support snapshots on your handler.