Consider extending the WoT `Thing` type

[benfrancis]

WebThings currently adds various proprietary top level members to WoT Thing Descriptions.

It would be good to document these as schemas so that WoT Consumers can understand them better, and they can potentially be used with JSON-LD prefixes in the future.

Proprietary members of a Thing used by WebThings Gateway that are likely to stick around include:

• floorplanX
• floorplanY
• layoutIndex
• selectedCapability
• groupId (currently group_id)

Also being added in WebThings Cloud:

• buildingId
• floorCode

See also: WebThingsIO/gateway#2832