Used to add children without native scrolling capabilities.
This is simply a convenience function; it is equivalent to adding the
unscrollable child to a viewport, then adding the viewport to the
scrolled window. If a child has native scrolling, use
add() instead of this function.
The viewport scrolls the child by moving its
GdkWindow, and takes the size of the child to be
the size of its toplevel GdkWindow. This will be
very wrong for most widgets that support native scrolling; for example,
if you add a widget such as GtkTreeView with a
viewport, the whole widget will scroll, including the column headings.
Thus, widgets with native scrolling support should not be used with
the GtkViewport proxy.