Something along the lines of add_sized() etc. functions in gtkiconfactory.c 
should maybe be exported for convenience. The hard part is deciding which of 
the match criteria in GtkIconSource to strip out of the convenience API. 
Other than stripping the match criteria, the other way to make things more
convenient would be to require inline pixbuf data, so people don't have to
create pixbufs, and possibly eliminate the GtkIconFactory* arg by using
some default icon factory.