Enhancement - undo

- Now creates/opens/closes undo.data file.
- All new tasks are logged in undo.data file.

src/TDB.h

@@ -64,6 +64,7 @@ public:
 
 private:
   FILE* openAndLock (const std::string&);
+  void writeUndo (const Task&, FILE*);
 
 private:
   std::vector <Location> mLocations;