Returns the horizontal and vertical extent of the decorator. The extent is
better thought of as the relative size of an object's bounding rectangle,
independent of where that widget may be placed on the display. The extent is
returned in a WidgetExtent
data structure defining the width and height of the
decorator. Note that this is the _actual_ extent of the widget as it will
appear on the display, as opposed to the preferred extent, which is the size
the object would choose to be under ideal circumstances.