screensharing: add framerate preference

src/media/video/v4l2/video_device_impl.cpp

src/media/video/v4l2/video_device_monitor_impl.cpp

src/media/video/video_device.h

src/media/video/video_device_monitor.cpp

src/media/video/video_input.cpp

src/media/video/winvideo/video_device_impl.cpp

src/media/video/winvideo/video_device_monitor_impl.cpp