WIP: Add support for XEP-0390: Entity Capability 2.0

[adiaholic]

[vanitasvitae]

Nice! I see you are not yet fed up with Smack development :+1:

I guess you are going in the right direction. I'd implement full ecaps2 first and then see how to unify ecaps2 and caps1 under a common API that ideally is not too different from caps1. Maybe you can "hide" ecaps2 and caps1 under the current caps1 API even, that would give you the benefits from ecaps2 while users can reuse the same API they are already used to.