fix filedownload on maximized view and context menu

- move the logic for remembering local filenames from
  messagecontentmodel to neochatroom
- use it also when maximized and in context menu opening, to stop
  re-downloading
- make onFileTransferCompleted signal connection one-shot (stops memory
  leak)

src/models/messagecontentmodel.h

@@ -141,6 +141,4 @@ private:
 
     void updateItineraryModel();
     bool m_emptyItinerary = false;
-
-    Quotient::FileTransferInfo fileInfo() const;
 };