I've reworked internals to use CallKit UI (native UI for incoming calls) which will allow us to, later on, answer calls while Siskin is in the background. For now, I've switched to CallKit, added support for answering Jingle Message Initiation (so that ie. Conversation could initiate a call with Siskin), tested solution to make sure that they work properly.

There is still a need to add support for receiving push notifications for an incoming call while offline and to use JMI for initiating a call.