Fix setting the wallpaper in XFCE

Fix setting the wallpaper in XFCE

* lisp/image/wallpaper.el  (wallpaper-command-args)
(wallpaper-default-set-function): Support new format specifiers
%S for screen, %W for workspace, and %M for monitor.
(wallpaper--default-setters): Use above new specifiers for XFCE.
(wallpaper--format-arg): New defun broken out from...
(wallpaper-default-set-function): ...here.
(wallpaper--get-height-or-width): Support noninteractive use.
* test/lisp/image/wallpaper-tests.el (wallpaper--format-arg/filename)
(wallpaper--format-arg/filename-hex)
(wallpaper--format-arg/width, wallpaper--format-arg/screen)
(wallpaper--format-arg/monitor, wallpaper--format-arg/workspace):
New tests.