Add CAP_PROP_STREAM_OPEN_TIME

* Added CAP_PROP_STREAM_OPEN_TIME to videoio module - can be used to query the time at which the stream was opened, in seconds since Jan 1 1970 (midnight, UTC). Useful for RTSP and other live video where absolute timestamps are needed. Only applicable to ffmpeg backends

* use nanoseconds instead of seconds to mark the stream open time, and change the cap prop name to CAP_PROP_STREAM_OPEN_TIME_NSEC

* use microseconds for CAP_PROP_STREAM_OPEN_TIME (nanoseconds rolls over too soon, and milliseconds/seconds requires a division)

* fix whitespace issue