Set XFCE wallpaper also in single-workspace-mode

Set XFCE wallpaper also in single-workspace-mode

This fixes setting the wallpaper on XFCE whether or not the
"/backdrop/single-workspace-mode" setting is true or false.
That XFCE setting controls whether or not the same wallpaper is used
on all workspaces or not.

* lisp/image/wallpaper.el (wallpaper-setter)
(wallpaper-command-args, wallpaper-default-set-function): Allow
using a function for getting the command line arguments.
(wallpaper-xfce-command-args): New function.
(wallpaper--default-setters): Use above new function for XFCE.