v2.5.0

1/ API change: gp_log_add_func callback is passed str instead of bytes
2/ Add some GPPort methods. This allows a port to be reset when an error
occurs.
3/ Pass CameraFile to gp_camera_capture_preview & gp_camera_file_get.
Optional - one will be created if needed.
4/ Remove Context::camera_autodetect method deprecated 3 years ago.
5/ Rename Context, PortInfo & PortInfoList. Add aliases of old names.
6/ Update libgphoto2 to v2.5.31.