Load localization catalog for slideshow config window

Summary: It was reported by Victor Ryzhykh that this patch fixes localization of the wallpaper slideshow config window.

Test Plan:
1. Open the config window in some ~100%-translated locale (ca, pt, sv, uk).
2. All messages should be translated.

Reviewers: #plasma, #localization, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26584