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: {F8286875}

Reviewers: #vdg, #plasma, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

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