Refactor some concepts of 'call' to 'conversation' to better reflect the conversation window

The window we currently open as a call is becoming more of a coversation window. Therefore I'd like to do two things: firstly renaming the talkilla.call-start event to talkilla.converstion-open, and secondly reworking the worker so that currentCall is now currentConversation which is handled by a Conversation object.

This should give clearer indications for future development about what is actually happening in these areas.