Undo

- Stubbed the undo implementation in TB2::revert (undo as a name is
  already taken), and in the process lose the capability.
- Add detection of <modifications> when running the 'undo' command
  and generate an error.

src/TDB2.h

@@ -97,6 +97,7 @@ public:
   void modify (const Task&);
   void commit ();
   void synch ();
+  void revert ();
   int  gc ();
   int  next_id ();