page.func, this function will
be used to determine what will be the next page when the user presses
the forward button. Setting page.func to NULL will make the assistant
to use the default forward function, which just goes to the next visible
page.gtkAssistantSetForwardPageFunc(object, page.func, data)objectpage.funcGtkAssistantPageFunc, or NULL to use the default one. [ allow-none ]datapage.func