This will allow user interfaces to more gracefully handle conversations with

disconnected accounts and offline conversations.
I didn't create a purple_conversation_new_federated as we don't have a purple_conversation_new yet. I'm waiting on both of those until PurpleConversation is a final type.

Compiled and ran the unit tests only. Writing unit tests for this right now would require a custom PurpleProtocol as well as a custom PurpleConnection and maybe more. So I skipped for the time being as this is pretty straight forward.