[wallpapers/image] Port wallpaper plugins (mostly) to QQC2

Summary:
BUG: 400549
FIXED-IN: 5.15.0

Test Plan:
On the lock screen KCM at 1.2x scale, before: {F6554037}

After: {F6554040}

All functionality continues to work on the lock screen and SDDM KCMs and desktop containment window.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18343