Enhancement - undo

- Added logging of both new tasks, and modified tasks.
- Added size of undo.data file to the statistics report.

src/TDB.h

@@ -65,6 +65,7 @@ public:
 private:
   FILE* openAndLock (const std::string&);
   void writeUndo (const Task&, FILE*);
+  void writeUndo (const Task&, const Task&, FILE*);
 
 private:
   std::vector <Location> mLocations;