Make wxDataViewModel::GetAttr() and GetAttrByRow() const.

This is an incompatible change but having to use a non-const model pointer to
call a clearly logically const version was simply too ugly so change it while
we still can.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62500 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/dataview/mymodels.cpp

@@ -433,7 +433,7 @@ void MyListModel::GetValueByRow( wxVariant &variant,
 }
 
 bool MyListModel::GetAttrByRow( unsigned int row, unsigned int col,
-                                wxDataViewItemAttr &attr )
+                                wxDataViewItemAttr &attr ) const
 {
     switch ( col )
     {