New library wallpaper.el pushed to master

[Stefan Kangas <stefankangas@gmail.com>]

Stefan Kangas <stefan@marxist.se> writes:

> I'm looking at teaching image-dired.el to set your desktop background.
> Because A) this finishes a TODO that has been in that file for going on
> 15 years, and B) any respectable image viewer must know how to set your
> desktop background.
>
> However, this code is not really tied to image-dired.el; only one line
> and the name of the defun ties it to that library.  For example, it
> could be used in thumbs.el (that has similar capabilities), and it could
> even be useful as a stand-alone command.
>
> So I started thinking that just like the recently added gravatar.el,
> perhaps we should put this stuff in a new library
> lisp/image/desktop-background.el?

I have now pushed the new library wallpaper.el and changed thumbs.el and
image-dired.el to use it.  It has the new command `wallpaper-set', and
the user option `wallpaper-commands' to control it.

I don't expect it to not work on anything but GNU/Linux for now, but
patches and suggestions are obviously very welcome.  (I hope that it
will be easy to add checks for different environments by adding new
`cl-defgeneric's.)

Thanks to Tassilo Horn for useful pointers in the previous discussion on
how to handle Sway/Wayland and Gnome.