Merge branch 'dvc-compare-values'

Fix comparing items with checkboxes in wxTreeListCtrl and make it
simpler to correctly implement item comparison in other
wxDataViewCtrl-derived classes.

See https://github.com/wxWidgets/wxWidgets/pull/558

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