Task: Remove std::map inheritance, clean up interface

- Make the Task object's interface more explicit by removing the
  std::map inheritance.
- Using this more explicit interface, remove unneeded ctors in order to
  allow the compiler to "Do The Right Thing"(tm).
  This leads to a performance improvement of 12% in the "add"
  performance test, and 7% for "import".

src/commands/CmdDenotate.cpp

@@ -124,7 +124,7 @@ int CmdDenotate::execute (std::string&)
       }
     }
 
-    if (before != task)
+    if (before.data != task.data)
     {
       std::string question = format (STRING_CMD_DENO_CONFIRM,
                                      task.identifier (true),