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/mymodels.h

@@ -188,6 +188,14 @@ private:
 class MyListModel: public wxDataViewVirtualListModel
 {
 public:
+    enum
+    {
+        Col_EditableText,
+        Col_IconText,
+        Col_TextWithAttr,
+        Col_Max
+    };
+
     MyListModel();
 
     // helper methods to change the model
@@ -202,12 +210,12 @@ public:
 
     virtual unsigned int GetColumnCount() const
     {
-        return 3;
+        return Col_Max;
     }
 
     virtual wxString GetColumnType( unsigned int col ) const
     {
-        if (col == 1)
+        if (col == Col_IconText)
             return wxT("wxDataViewIconText");
 
         return wxT("string");