Rework ActionsHandler as static helper functions.
The functions are now invoked from ChatBarCache so there is no need to pass an actions handler object around qml simplifying the code.
Move reply pane to use MessageContentModel. This means the reply pane component is no longer required.
This commit also limits the size of code and image componets in a reply to keep them from getting too huge.
So at the moment this remains behind the feature flag as this only adds a threadmodel and a basic visualisation. There is much more to come to get it ready for full release.
