Use the default Plasma wallpaper on the login screen

Summary:
As discussed and agreed to by VDG (T7914), change the SDDM theme background from the blue color to the current Plasma wallpaper.

BUG: 381407
FIXED-IN: 5.13.0

Test Plan:

• Build and deploy plasma-workspace with this patch
• Log out

Reviewers: Plasma, VDG, broulik, abetts, ngraham, mart

Reviewed By: Plasma, VDG, abetts, ngraham, mart

Subscribers: mart, davidedmundson, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11309