The length (in bytes) of the file being downloaded.
This will initially be -1, and will be populated as soon as the appropriate header is received from the server. Its value
will never change after this.
Note that change notifications for this property (#GObject::notify emissions) may be emitted in threads other than the one which created the DownloadStream. It is the client's responsibility to ensure that any notification signal handlers are either multi-thread safe or marshal the notification to the thread which owns the DownloadStream as appropriate.