Add full, asynchronous handling of subscription requests
Created by: LNJ2
The handling of incoming subscription requests can be done in QML, now.
First the signal of an incoming subscription request is emitted ( kaidan.subscriptionRequestReceived(from, message)), then the use can decide if they want to accept, decline or ignore the request. If they want to decline or accept it, the signal kaidan.subscriptionRequestAnswered(jid, accepted) should be emitted (accepted=true: allow, accepted=false: decline).