When a new message is delivered, the conversation list should update by changing the preview text and re-sorting the conversations
Bonus bug discovered and fixed: previously, when the conversations list was being populated, it made a request for the first message in every conversation. This would be fine if the conversationdbusinterface pulled from local cache. However, this actually triggers a request to the phone for *every* conversation.
This should be handled differently in conversationdbusinterface's requestConversation as well, but that's a project for a later day (TODO comments added)