Custom Handlers for Listers: 'snapshot'

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

Its arguments are:-

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

Unlike  snapshot for AppIcons , the lister's position is not sent with the
event and it is up to you to get it using  lister query position .

It is also up to you to save the information for later use, if you wish to
support snapshots on your handler.