Sets the page forwarding function to be 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.
| this |
a `GtkAssistant` |
| page_func |
the `GtkAssistantPageFunc`, or null to use the default one |
| data |
user data for |
| destroy |
destroy notifier for |