OIC spec: OicDeviceChangeEvent field "type" needs to be renamed to "changeType"

[gabrielschulhof]

@zolkis The generic Event structure already has a field named type, referring to the type of event. This allows a common handler for multiple event types. Thus, the type field for the OicDeviceChangeEvent must be set to "devicechange", and the actual OicChangeType must be stored in a different field. I propose we name that field changeType.

[gabrielschulhof]

https://github.com/solettaproject/soletta/blame/master/doc/js-spec/oic.md#L297