Interface for a drill-down viewable object. Such an object has
content that may or may not be displayed, at the discretion of the
viewer. The content is returned to the viewer as an array or as an
Iterator, and the object provides a clue as to which technique the
viewer should use to get its content.
A POIFSViewable object is also expected to provide a short
description of itself, that can be used by a viewer when the
viewable object is collapsed.