co2_sensor: Handle pre-calibration by states

* split state PREPARECALIBRATION into
PREPARECALIBRATION_INSTABLE (co2 measurements are changing
too much to start calibration) and

PREPARECALIBRATION_STABLE (co2 measurements are stable,
now waiting for 60 stable measurements)

* Handle all possible LED signal states in showState()
* drop now unused `waiting_color`
1 job for dev/sensorstates in 2 minutes and 5 seconds (queued for 1 second)