Determines whether the print operation may run asynchronously or not.
Some systems don't support asynchronous printing, but those that do will return gtk.types.PrintOperationResult.InProgress as the status, and emit the gtk.print_operation.PrintOperation.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.types.PrintOperationAction.Export support asynchronous operation.
Set allowAsync property.