Task

- Implemented an overloaded ::modify method that walks the parse tree instead
  of iterating over an A3 list.
- ::modify takes a Task::modType argument, which specifieѕ how words are
  handled (replace description, prepend to description, append to description,
  add annotation), and this eliminates the need for multiple modify methods.
- Marked existing methods as obsolete.

src/Task.h

@@ -154,9 +154,14 @@ public:
   float urgency_c () const;
   float urgency ();
 
+  // TODO Obsolete.
   void modify (const A3&, std::string&);
+  // TODO Obsolete.
   bool next_mod_group (const A3&, Arg&, unsigned int&);
 
+  enum modType {modReplace, modPrepend, modAppend, modAnnotate};
+  void modify (modType);
+
 private:
   int determineVersion (const std::string&);
   void parseJSON (const std::string&);