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.
Use 'mode' enum definition from gstcamerabin-enum file to avoid
conflicts between v4l2camerasrc and gstcamerabin2 modes.
For now there is a MODE_PREVIEW there that is only used on the
camerasrc, not sure if we are keeping it at the future, but for
now this works.
