Use symbolic constants for list model columns in dataview sample.

Using Col_EditableText, Col_IconText and Col_TextWithAttr instead of 0, 1 and
2 makes the sample code a bit easier to read.

Also use switch on the column value instead of nested ifs everywhere to give
compiler a chance to warn us if we forget to update some function when a new
column is added.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62587 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/dataview/dataview.cpp

@@ -545,10 +545,17 @@ void MyFrame::BuildDataViewCtrl(wxPanel* parent, unsigned int nPanel, unsigned l
             m_ctrl[1]->AssociateModel( m_list_model.get() );
 
             // the various columns
-            m_ctrl[1]->AppendTextColumn("editable string", 0, wxDATAVIEW_CELL_EDITABLE);
-            m_ctrl[1]->AppendIconTextColumn("icon", 1, wxDATAVIEW_CELL_EDITABLE);
+            m_ctrl[1]->AppendTextColumn("editable string",
+                                        MyListModel::Col_EditableText,
+                                        wxDATAVIEW_CELL_EDITABLE);
+            m_ctrl[1]->AppendIconTextColumn("icon",
+                                            MyListModel::Col_IconText,
+                                            wxDATAVIEW_CELL_EDITABLE);
             m_ctrl[1]->AppendColumn(
-                new wxDataViewColumn("attributes", new wxDataViewTextRenderer, 2 ));
+                new wxDataViewColumn("attributes",
+                                     new wxDataViewTextRenderer,
+                                     MyListModel::Col_TextWithAttr)
+            );
         }
         break;