v4l2camerasrc: Implement video capture

v4l2camerasrc: Implement video capture

Implements video capture on v4l2camerasrc by using the mode property,
when mode is set to video, the pad probe pushes a new segment
and starts pushing buffers on the pad, when it the property is
sent back to preview, the pad probe pushes an EOS and stops
pushing buffers.

This is controlled by a Recording State variable, that is protected
by the GST_OBJECT_LOCK. I don't think locking for every buffer is
nice, so we could find an alternative lockless way here.