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