This was changed in 9ed5224470 but this
ended up causing all sorts of strange formatting issues like the quick
format bar not working, and pastes from other applications that kept the
formatting.
Update `ChatDocumentHandler` so `RoomManger` is no longer required for saving the text in the chatbar between room switches. This is achieved by allowing `ChatDocumentHandler` to get the correct `ChatBarChache` itself rather than having to have it passed from `ChatBar.qml`. This avoids any race conditions.
