* gtk/gtkcellrendererpixbuf.c (gtk_cell_renderer_pixbuf_set_property):
make sure that setting any of pixbuf/stock-id/icon-name resets the
others because they are mutually exclusive, and that unsetting any
of them only resets the pixbuf and nothing else. Also added
some missing g_object_notify().
(gtk_cell_renderer_pixbuf_get_property): simplified calls to
g_value_set_object().