Determines whether the print operation may run asynchronously or not.
Some systems don’t support asynchronous printing, but those that do
GTK_PRINT_OPERATION_RESULT_IN_PROGRESS as the status, and
GtkPrintOperation::done signal when the operation
is actually done.
The Windows port does not support asynchronous operation at all (this
is unlikely to change). On other platforms, all actions except for
GTK_PRINT_OPERATION_ACTION_EXPORT support asynchronous operation.