Re-factor out un-used invisible properties from the classifier #334
Description
Internally the Zigbee adapter adds a lot of properties with a '_' prefix in their name which used to be exposed in Thing Descriptions with a visible member set to false (so that they were hidden in the UI). In recent releases those properties are just omitted from the Thing Description altogether since visible is not a W3C standard member of a property affordance in a Thing Description.
Some of these invisible properties contain potentially useful metadata like minimum and maximum values which could be expressed in a Thing Description in another way.
It would be good to refactor the classifier and for each hidden property either remove it (since they're not actually used anywhere any more), or include the information in the Thing Description in a different way.
For now these invisible properties are just kept internal to the adapter since the gateway no longer honours the "visible" state.