Use symbolic constants for list model columns in dataview sample.

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