Refactor system specific code in wallpaper.el

Refactor system specific code in wallpaper.el

* lisp/image/wallpaper.el (wallpaper-set-function): New defvar
containing system specific function for setting wallpaper.
(wallpaper-default-set-function): Factor out function from...
(wallpaper-set): ...here.  Use above new defvar.

(wallpaper-default-file-name-regexp): Delete defvar.
(wallpaper-image-file-extensions): New defvar.
(wallpaper--image-file-regexp): New defun that returns a regexp to
match for completion purposes.

(wallpaper--use-default-set-function-p): New defun.
(wallpaper--find-command, wallpaper--find-command-arguments):
Do nothing on MS-Windows and Haiku.