skel: add xfce4-desktop wallpaper list (cc0-wallpapers)

config/includes.chroot/etc/skel/.config/xfce4/desktop/backdrop.list

@@ -0,0 +1,6 @@
+# xfce backdrop list
+/usr/share/backgrounds/cc0-wallpapers/unsplash1.jpg
+/usr/share/backgrounds/cc0-wallpapers/unsplash2.jpg
+/usr/share/backgrounds/cc0-wallpapers/unsplash5.jpg
+/usr/share/backgrounds/cc0-wallpapers/unsplash6.jpg
+/usr/share/backgrounds/cc0-wallpapers/unsplash10.jpg