according to the pygtk function for create_items, if the item 
is an <ImageItem>, the extra data should be either a string 
or a pixbuf.  

the C create_items function actually only appears to take inline data
(it internally creates a pixbuf from it).

attached is a patch against 2.0.0 to make it function correctly 
if passed a pixbuf.  the pixbuf is first serialized and then passed
to the C function.  the memory created is then owned by the itemfactory's
internal pixbuf.