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/db/datastore.h

@@ -120,6 +120,9 @@ namespace BlackMisc
             //! Convert string to DB key
             static int stringToDbKey(const QString &candidate);
 
+            //! The key index
+            static const CPropertyIndex &keyIndex() { static const CPropertyIndex k(IndexDbIntegerKey); return k; }
+
         protected:
             //! Constructor
             IDatastoreObjectWithIntegerKey() {}
@@ -201,6 +204,9 @@ namespace BlackMisc
             //! Invalid key
             static QString invalidDbKey() { return {}; }
 
+            //! The key index
+            static const CPropertyIndex &keyIndex() { static const CPropertyIndex k(IndexDbStringKey); return k; }
+
         protected:
             //! Constructor
             IDatastoreObjectWithStringKey() {}

src/blackmisc/propertyindexlist.cpp

@@ -20,4 +20,12 @@ namespace BlackMisc
         CSequence<CPropertyIndex>(other)
     { }
 
+    CPropertyIndexList CPropertyIndexList::copyFrontRemoved() const
+    {
+        if (this->size() < 2) { return CPropertyIndexList(); }
+        CPropertyIndexList copy(*this);
+        copy.pop_front();
+        return copy;
+    }
+
 } // namespace

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