Ref T529, deterministic sort order when column values are equal

* the Qt model sorts by column
* when multiple values have the same column value the order among those is more or less random
* added additional property indexes to determine the order among the equal values

src/blackmisc/propertyindexlist.h

@@ -35,6 +35,9 @@ namespace BlackMisc
 
         //! Construct from a base class object.
         CPropertyIndexList(const CSequence<CPropertyIndex> &other);
+
+        //! List without front element, or empty list if not applicable
+        CPropertyIndexList copyFrontRemoved() const;
     };
 } //namespace