Message Content Rework

For now everything should look identical. However this moves to using a model for the content of the message and is intended to lay the foundation for improved message content representation, e.g. splitting up a text message in multiple sections and using different delegates for things like code and quotes.

src/chatbarcache.h

@@ -186,7 +186,7 @@ public:
 
 Q_SIGNALS:
     void textChanged();
-    void relationIdChanged();
+    void relationIdChanged(const QString &oldEventId, const QString &newEventId);
     void threadIdChanged();
     void attachmentPathChanged();