plugins/wallpaper: support wayland and fix indentation

[darukutsu]

nitrogen & pywal don't set background on wayland. Therefore add swww.

[jarun]

Thank you!

[jarun]

@darukutsu if you can test Wayland, are there any other plugins which need changes to work on Wayland?

[darukutsu]

@jarun everything looks ok.
The only plugin which behaves incorrectly is preview-tabbed.
Universal solution for wayland would be using wlrctl
It only works with urxvt and need to add line

previewer_loop() {
...
    wlrctl window focus title:nnn
    
    # Bruteforce focus stealing prevention method,
    # works well in floating window managers like ...
...
}

It's not suitable since user may have opened multiple instances with nnn, but other than that there is only compositor-specific solutions. I don't think it's needed if using something like preview-tui with kitty.

Also today discovered that alacritty supports XEmbed so maybe add to preview-tabbed

elif type alacritty >/dev/null 2>&1 ; then
    TERMINAL="alacritty --embed"