Let ListBox column sorting be cleared

Once a ListBox column has been sorted, there is no way to remove the sort direction. SortedColumnIndex and SortAscending are read-only, Sort() always sets a direction, and header clicks only toggle between ascending and descending. The only action that clears the indicator today is manually drag-reordering a row.

Add a third "no sort" state so a column's sort can be removed:

• New SortDirection property (ListBoxSortDirection.None / Ascending / Descending) reporting the current state, with None when unsorted.
• New ClearSort() method that removes the sort indicator while keeping the rows in their current order and selection, regardless of ColumnSortable.
• New opt-in ColumnSortCycleToNone property (default False). When ColumnSortable is True and this is enabled, clicking the active sorted column's header cycles ascending → descending → no sort. Existing two-state click behaviour is unchanged by default.

Works identically for desktop and web ListBox controls.