A CellRendererPixbuf can be used to render an image in a cell. It
allows to render either a given Pixbuf (set via the pixbuf property) or a
stock icon (set via the stock-id property).
To support the tree view, CellRendererPixbuf also supports rendering
two alternative pixbufs, when the is-expander property is True. If the
is-expanded property is True and the pixbuf-expander-open property is set
to a pixbuf, it renders that pixbuf, if the is-expanded property is False
and the pixbuf-expander-closed property is set to a pixbuf, it renders that