Introduce libavdevice to make v4l2 available to the ffmpeg backend
* introduce libavdevice to make v4l2 available to the ffmpeg backend
* downgrade the min required libavdevice version to 53.2.0
* make libavdevice optional
* create OCV_OPTION OPENCV_FFMPEG_ENABLE_LIBAVDEVICE and add definition through ocv_add_external_target
* move OCV_OPTION 'OPENCV_FFMPEG_ENABLE_LIBAVDEVICE' to detect_ffmpeg.cmake