Clean up Message/RosterController
I separated them again, so the RosterController will also connect to the Message received event, and will upsate the unread message counter on its own. I also removed many `emit xxxModelChanged`. They are unnecessary and I don't know why I added them at all (in these cases). There were also some unused #includes, that I have removed. But the QML API has also changed through this and sendMessage and chatPartner are now on the top level of `kaidan`.
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