TDB2

- Implemented CmdAdd.cpp and CmdLog.cpp using TDB2.
- Implemented simple append writes in TDB2.
- Modified CmdImport to accept and parse JSON.
- Added more const-ness in DOM, Expression and Task, to allow TDB2::get_tasks
  to return a const vector ref, which is a Very Good Thing.
- Corrected usage for the export command.
- Implemented Task::urgency as a call to Task::urgency_c, which is a const
  overload to allow urgency calculations (without caching) for const Task
  objects.
- Removed obolete code from TDB.
- Added lots of diagnostic output for TDB2 - it's annoying, but will be
  gone soon.
- Added mention in CmdHelp of the new <filter> and <modifications>
  syntax elements.  Needs more.
- Added Command::filter overload which uses TDB2.  Not in use yet.

src/TDB2.h

@@ -45,9 +45,9 @@ public:
 
   void target (const std::string&);
 
-  const std::vector <Task>& get_tasks ();
+  const std::vector <Task>&        get_tasks ();
   const std::vector <std::string>& get_lines ();
-  const std::string& get_contents ();
+  const std::string&               get_contents ();
 
   void add_task (const Task&);
   void modify_task (const Task&);
@@ -55,7 +55,6 @@ public:
   void clear_lines ();
   void commit ();
 
-public:
   void load_tasks ();
   void load_lines ();
   void load_contents ();
@@ -68,7 +67,6 @@ public:
   bool _loaded_contents;
   std::vector <Task> _tasks;
   std::vector <Task> _added_tasks;
-  std::vector <Task> _removed_tasks;
   std::vector <Task> _modified_tasks;
   std::vector <std::string> _lines;
   std::vector <std::string> _added_lines;
@@ -87,7 +85,8 @@ public:
   void add (const Task&);
   void modify (const Task&);
   void commit ();
-  int gc ();
+  int  gc ();
+  int  next_id ();
 
   void dump ();
   void dump_file (ViewText&, const std::string&, TF2&);
@@ -101,6 +100,7 @@ public:
 
 private:
   std::string _location;
+  int _id;
 };