Add wallpapers for other desktops

[yselkowitz]

Per discussions today, we want to find a way to create the symlinks needed for other desktops in a systematic fashion that can easily be added to each EL distro's own logos/backgrounds packages, so that it can be used by default in EPEL-shipped distros, starting with KDE.

• Create /usr/share/backgrounds/default.(jxl|png|xml) etc. to replace Fedora's desktop-backgrounds-compat for Xfce/MATE/etc.
• Create a structure under /usr/share/wallpapers for KDE/LXQt.