Implement sorting in wxTreeListCtrl.

Implement sorting in wxTreeListCtrl.

Allow the user to sort the control contents by clicking on the columns with
wxCOL_SORTABLE flag and also provide SetSortColumn() method to sort the
control programmatically.

Also add wxTreeListItemComparator class and SetItemComparator() method to
allow customizing the way the items are compared.

Update the sample to show how to define a custom comparator.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69176 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775