This class is an abstract superclass for widgets offering the ability
to pick items from a pre-built list. The list is usually hidden, but
a button on the right of this widgets makes it pop up. This widget
is thus composed of three parts: an unspecified text widget (shown on
the left of the button and always visible), the button widget (shown
on the right, it depicts a down arrow, and is always visible), and
the pop-up list widget.