And reverse menu order in order to avoid it reading "Change Background… Settings"
"System Settings" shouldn't be used because it's not used to mean the control-center in any other place.
https://bugzilla.gnome.org/show_bug.cgi?id=694290
BackgroundMenu must ensure the actor it attaches to is reactive, and
the layout manager must create a background menu for the first background
too.
https://bugzilla.gnome.org/show_bug.cgi?id=694227