Rewrite wallpaper.el to use a cl-defstruct

Rewrite wallpaper.el to use a cl-defstruct

* lisp/image/wallpaper.el
(wallpaper--default-commands): Delete variable.
(wallpaper-setter): New cl-defstruct.
(wallpaper--default-methods-create): New macro.
(wallpaper--default-setters):
(wallpaper--current-setter): New variables.
(wallpaper--find-setter): New defun to pick a wallpaper-setter.
(wallpaper--find-command, wallpaper--find-command-args):
Use 'wallpaper--find-setter'.
(wallpaper-command): Doc fix.

* test/lisp/image/wallpaper-tests.el
(wallpaper--find-command/return-string)
(wallpaper--find-command-args/return-list)
(wallpaper--image-file-regexp/return-string): New tests.