Task Refactoring

- Task is no longer a map of string to Att.  Att is itself a name/
  value pair, so the name was redundant.  Task is now a map of string
  to string.  This brings the obsoletion of Att much closer.

src/TDB.cpp

@@ -836,8 +836,7 @@ void TDB::undo ()
     view.addRow ();
 
     // Add rows to table showing diffs.
-    std::vector <std::string> all;
-    Att::allNames (all);
+    std::vector <std::string> all = context.getColumns ();
 
     // Now factor in the annotation attributes.
     Task::iterator it;