make wallpaper configurable non-interactively

[ctr49]

Gnome, Xfce, KDE all support modifying the current wallpaper through some sort of interface (see https://www.baeldung.com/linux/change-desktop-wallpaper-from-terminal). While we are waiting for some of the features requested in #70 #69 #34 and others it may be worthwhile to make it configurable externally so workarounds may be implemented to get it done.

[mmstick]

This is already possible through the cosmic-config file or the wallpaper portal