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.
Adds mode property to camerabin2, allowing users to
select between video and stills capture. Also adds
start/stop capture actions to trigger and stop
capturing
