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),

src/commands/CmdImport.cpp

@@ -205,7 +205,7 @@ void CmdImport::importSingleTask (json::object* obj)
     if (hasGeneratedEnd)
       task.set ("end", before.get ("end"));
 
-    if (before != task)
+    if (before.data != task.data)
     {
       CmdModify modHelper;
       modHelper.checkConsistency (before, task);

src/commands/CmdModify.cpp

@@ -76,7 +76,7 @@ int CmdModify::execute (std::string&)
     Task before (task);
     task.modify (Task::modReplace);
 
-    if (before != task)
+    if (before.data != task.data)
     {
       // Abort if change introduces inconsistencies.
       checkConsistency(before, task);

src/commands/CmdUDAs.cpp

@@ -137,7 +137,7 @@ int CmdUDAs::execute (std::string& output)
   std::map <std::string, int> orphans;
   for (auto& i : filtered)
   {
-    for (auto& att : i)
+    for (auto& att : i.data)
       if (att.first.substr (0, 11) != "annotation_" &&
           context.columns.find (att.first) == context.columns.end ())
         orphans[att.first]++;