Add "Safe Landing" wallpaper

Summary:
This patch adds the Safe Landing wallpaper from our recent wallpaper competition:
https://forum.kde.org/viewtopic.php?f=312&t=160777

The author provided me his name and email address in a private forum message since the
subforum with the original thread has since been locked. He agreed to change the license
to LGPLv3 as well.

In the interests of space, I've opted to add the JPEG version because it looks better than
the dithered PNG version, and the un-dithered version is too large.

Test Plan:

Reviewers: VDG

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21609