Add "Volna" wallpaper

Summary:
The default is being replaced with "Flow" in D29449, so we move the old default "Volna"
to this repo.`

Depends on D29449

Test Plan:

Reviewers: VDG, Plasma, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29448