[Jbeta] Gtk demos broken

[Sherlock, Ric]

I was trying out the Gtk demos gui/gtk/demo/tut*.ijs
But I think that the event-handling mechanism has evolved since they were 
created and so the gtkcallback verb causes a value error. For those of us 
coming to grips with Gtk, it would be a big help to have the demos working.
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