[plasma-workspace-wallpapers] Add the Elarun wallpaper

Summary: Elarun is an  iconic and beautiful KDE wallpaper that only takes up 1.4 MiB of disk space and as such would make a fine addition to our collection.

Reviewers: ngraham, #vdg, rooty

Reviewed By: ngraham, #vdg, rooty

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19872