Adds setWallpaper for sway desktop environment

[RomeoV]

Simple command line call for settings the downloaded picture for the
sway desktop environment.

[marguerite]

detect and set is a pair of work...are you sure “sway” can be automatically detected(not by the desktopenvironment variable in config)? or you should also contribute the sway detection codes

[RomeoV]

I use this systemd daemon where I can just pass sway as a flag:

[Unit]
Description=Bing Wallpaper of the Day as your Linux Desktop's wallpaper
After=network-online.target graphical-session.target
Wants=network-online.target

[Service]
Type=simple
ExecStart=%h/go/bin/linux-bing-wallpaper --daemon --desktop sway --dir %h/Pictures/Wallpapers
Restart=always
RuntimeMaxSec=3600

[Install]
WantedBy=default.target