[EVENTS] Design, implement and use in events the new mid-level entities

[johny-b]

1. We'd like to have an intermediate layer between events and things that are a direct interface to the REST API (yapapi.rest.*).
2. This is directly related to the other wide goal we consider pursuing - the new, convenient mid-level API. This should be the same objects.

Because of 2, this requires some serious planing/designing effort. Or at least we should be prepared to change this in the future.