GAppInfo that is registered as the default
application to handle the file specified by file.gFileQueryDefaultHandler(object, cancellable = NULL, .errwarn = TRUE)objectGFile to open.cancellableGCancellable object, NULL to ignore.cancellable is not NULL, then the operation can be cancelled by
triggering the cancellable object from another thread. If the operation
was cancelled, the error G_IO_ERROR_CANCELLED will be returned.