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.
