Make it easier to customize wallpaper command

Make it easier to customize wallpaper command

* lisp/image/wallpaper.el
(wallpaper--default-commands): New defvar.
(wallpaper--find-command): Use above new defvar.
(wallpaper--find-command-arguments): New defun.
(wallpaper-command): Rename from 'wallpaper-commands' and change
type to string.  Use 'wallpaper--find-command' to set it.
(wallpaper-command-args): New defcustom.  Use
'wallpaper--find-command-arguments' to set it.
(wallpaper--set-wallpaper-command): New defun.  Use as :set property
for 'wallpaper-command'.
(wallpaper-set): Use above new defcustoms to set the wallpaper.
Suggested by Eli Zaretskii <eliz@gnu.org>.