Note: Since Combos can contain both a list
and an editable text field, it is possible to confuse methods
which access one versus the other (compare for example,
clearSelection() and deselectAll()).
The API documentation is careful to indicate either "the
receiver's list" or the "the receiver's text field" to
distinguish between the two cases.
Note that although this class is a subclass of Composite,
it does not make sense to add children to it, or set a layout on it.
Note: Only one of the styles DROP_DOWN and SIMPLE may be specified.
IMPORTANT: This class is not intended to be subclassed.