#11 Add instead of replace covers

[Freek Dijkstra]

It would be great if clutter would *add* the picture to iTunes
instead of replacing what there already is. I don't mind if a song
has two images attached to it, and iTunes handles that very well.

I'm not entirelly sure if this interferes with request #965105. If
Clutter fetches the image from iTunes, and then again adds it, you
would end up with the same image twice. Perhaps this is no big
deal because the user just should have chosen "add to iTunes". But
what now if 2 of the 3 images did already contain the image, but
the fourth does not?