Summary:
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)

Test Plan:
 - Launch SMS app
 - Verify conversations list appears
 - Verify lack of massive stream of debug output indicating lots of messages for the wrong conversation are being received
 - Verify that opening a particular conversation shows the messages after a short delay while the backend fetches the content from the phone
 - Verify that receiving a new message into an existing conversation updates the conversation list

Reviewers: #kde_connect, nicolasfella

Reviewed By: #kde_connect, nicolasfella

Subscribers: nicolasfella, apol, kdeconnect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D15608