The first step of writing a new widget class is to decide the base
class from which the new class. Usually, if the new class does not
share any common features with other classes, it should be derived
from the TixPrimitive class. If it does share common features with
other classes, then it should be derived from the appropriate base
class. For example, if the new class support scrollbars, it should be
derived from TixScrolledWidget; if it displays a label next to its
``main area", then it should be derived from TixLabelWidget.
In the case of our new TixArrowButton class, it doesn't really share
any common features with other classes, so we decide to use the base
class TixPrimitive as its superclass.