TDB2

- Full implementation of TDB2::add, which includes pending/completed, undo
  and backlog.
- Removed obsolete TF2::commitUndo, which is no longer necessary.
- Added File::truncate method, to allow a TDB2 shortcut.

src/TDB2.h

@@ -54,7 +54,6 @@ public:
   void add_line (const std::string&);
   void clear_lines ();
   void commit ();
-  void commitUndo ();
 
   void load_tasks ();
   void load_lines ();