Returns the horizontal and vertical extent of the draw 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 draw decorator 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.