Add `emit_event` to `ThingServerInterface`

[julianstirling]

By adding emit_event to ThingServerInterface we can centralise event-related code that's now duplicated in multiple descriptors.

This would also enable setting properties at __init__ for a Thing.

This is based on a discussion with @rwb27 in #183 (comment)

[julianstirling]

@rwb27 It would be really good to get this in soonish

[rwb27]

This is (finally) included in #256 (as publish rather than emit_event).