Emitted when displaying the print dialog with webkit_print_operation_run_dialog().
The returned #WebKitPrintCustomWidget will be added to the print dialog and
it will be owned by the @print_operation. However, the object is guaranteed
to be alive until the #WebKitPrintCustomWidget::apply is emitted.
Emitted when displaying the print dialog with webkit_print_operation_run_dialog(). The returned #WebKitPrintCustomWidget will be added to the print dialog and it will be owned by the @print_operation. However, the object is guaranteed to be alive until the #WebKitPrintCustomWidget::apply is emitted.