Specifies the index of the selected item.
property ItemIndex: integer;
Read ItemIndex to determine which item is selected. The first item in the list has index 0, the second item has index 1, and so on.
Set ItemIndex programmatically to select an item by passing in the index value.
If new value less then 0 and ItemCount > 0 then ItemIndex will be 0,
Else ItemIndex will be set to new value.