Add a few scale related flags.

Add a few scale related flags.

 - SR_MQ_MASS: Mass, as measured by scales / balances.

 - SR_UNIT_*: Various units of mass.

 - SR_MQFLAG_UNSTABLE: A flag denoting that a value has not
   yet stabilized (settled). E.g. when placing an object on a scale
   it'll take a few moments until a stable reading is available.
   Measurement values marked with SR_MQFLAG_UNSTABLE denote that they
   are "unsettled", unstable values (not yet stabilized).
   The absence of SR_MQFLAG_UNSTABLE denotes that the value is stable.

 - SR_CONF_SCALE: A device class for weighing scales / balances.