Adds property that informs if v4l2camerasrc is available
for starting a new capture.
It is useful for applications to know (via deep-notify) when the
property changes and a new capture is possible. Note, however, that
starting a new capture from the notify callback will cause a deadlock.
Removes the old logic for v4l2camerasrc that used the mode
property switching to start/stop captures to make it identical
to camerabin2 behavior and to allow the future addition of
pausing a video recording.
This also removes the MODE_PREVIEW as it became useless.
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.
