[SDDM theme] Update the theme preview

Summary: The current preview of our Breeze SDDM theme is outdated. This patch adds a new and updated preview.

Test Plan:
Before:

After:

Reviewers: Plasma, VDG, Breeze, GB_2

Reviewed By: VDG, GB_2

Subscribers: GB_2, oussemabouaneni, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18012