Add wxDataViewModel::DoCompareValues() useful virtual hook

This new method, called from the default Compare() implementation, is
simpler to override in the derived classes than Compare() itself.

src/common/datavcmn.cpp

@@ -378,6 +378,12 @@ int wxDataViewModel::Compare( const wxDataViewItem &item1, const wxDataViewItem
         if (res != 0)
           return res;
     }
+    else
+    {
+        int res = DoCompareValues(value1, value2);
+        if (res != 0)
+            return res;
+    }
 
 
     // items must be different