Utility functions to sort text messages by timestamp (ascending/descending)

* compare function for text message
* resort function
* text message edit can display HTML text messages with ascending/descending timestamp
* made CTextMessageListModel -> CListModelTimestampObject

src/blackmisc/timestampbased.h

@@ -138,10 +138,10 @@ namespace BlackMisc
         //! Any of the timestamp indexes
         static bool isAnyTimestampIndex(int index);
 
-    protected:
         //! Can given index be handled
         static bool canHandleIndex(const CPropertyIndex &index);
 
+    protected:
         //! Constructor
         ITimestampBased();