More QML fidgetery

More QML fidgetery

While playing around with various approaches to how to implement ChatView in QML, I think slowly
things are shaping up. We now have extended the QmlMessageModel to provide all info needed for rendering
a QmlChatLine in one data stracture, in order to avoid numerous calls to the model whenever a ChatLine
is instantiated.

Currently there's a lot of code duplication with the related non-QML classes, but this will either go away
once we decide to kill the QGV-based ChatView, or be unified into a common set of base classes if we
find out that both implementations are going to stay around.