This adds a dependency on gsettings-desktop-schemas >= 2.91.92.
The version was chosen for the change from "picture-filename" to
"picture-uri" in the "org.gnome.desktop.background" schema.
Also, we now save the background image under ~/Pictures since the old
directory (~/.gnome2/wallpapers) is presumed deprecated in GNOME 3.
e076ca2f77