make TDB2.pending/completed non-public

References to methods on these fields are replaced with methods on TDB2,
several of which already existed.

src/TDB2.cpp

@@ -1133,6 +1133,18 @@ const std::vector <Task> TDB2::all_tasks ()
   return all;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+const std::vector <Task> TDB2::pending_tasks ()
+{
+  return pending.get_tasks ();
+}
+
+////////////////////////////////////////////////////////////////////////////////
+const std::vector <Task> TDB2::completed_tasks ()
+{
+  return completed.get_tasks ();
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Locate task by ID, wherever it is.
 bool TDB2::get (int id, Task& task)