Fixing a multi-monitor background regression

[Matthias Clasen <matthias clasen gmail com>]

In 2.28, we changed the handling of backgrounds on multi-monitor
setups to be repeated on each monitor instead of stretched over all of
them. While I believe that was the right call and the current
behaviour is much better for most of our users, there is a vocal group
of people who are very unhappy that their carefully crafted
dual-screen backgrounds no longer work.

https://bugzilla.gnome.org/show_bug.cgi?id=609809
https://bugzilla.gnome.org/show_bug.cgi?id=603551

have patches to gnome-desktop and control-center to bring spanning
backgrounds back as a style option in the appearance capplet (next to
'Tiled', 'Zoom', etc).

This is a borderline UI change (adds a new option to the combo box)
and adds one new string.
I think we should get this in for 2.30. Comments ?

Matthias