[Wallpaper] Allow adding multiple images at once from the file picker dialog

Summary:
BUG: 402223
FIXED-IN: 5.15.0

Test Plan: Added multiple images from the file dialog. They all showed up in the wallpaper grid.

Reviewers: #konsole, broulik, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17815