ComboBox class is a combination of a ListBox and an Edit Box. A ComboBox
either has an Edit attached to a showing ListBox (extended) or an Edit
attached to a pop-up ListBox (collapsed). This class is used for ComboBoxes
that are either extended or collapsed. Elements in a ComboBox are identified
by a zero-based identification number. The number corresponds to the
order, from top to bottom, in which they appear in the list.