cellrendererpixbuf: Store the texture in the iconhelper paintable

cellrendererpixbuf: Store the texture in the iconhelper paintable

That's kinda weird but allows us to delete the texture case from
GtkIconHelper and GTK_IMAGE_TEXTURE from the GtkImageType enum.

And it doesn't cause any other problems because the cell renderer
can't deal with paintables - otherwise it would mirror GtkImage and have
a "paintable" property instead.