Refactor task diffs to handle new attributes

This refactors task(Info)Differences to be methods of Task and to
correctly handle the `annotation_`, `tags_`, and `dep_` attributes.

src/commands/CmdDone.cpp

@@ -98,7 +98,7 @@ int CmdDone::execute (std::string&)
           task.addAnnotation (Context::getContext ().config.get ("journal.time.stop.annotation"));
       }
 
-      if (permission (taskDifferences (before, task) + question, filtered.size ()))
+      if (permission (before.diff (task) + question, filtered.size ()))
       {
         updateRecurrenceMask (task);
         Context::getContext ().tdb2.modify (task);