[sddm-theme/lock screen] Move main stack to a more vertically centered position
Test Plan:
Lock screen - before:
and after:
Login screen - before:
and after:
Last login screen patch, sorry it took so long.
Reviewers: VDG, ngraham, filipf, davidedmundson
Reviewed By: VDG, ngraham
Subscribers: plasma-devel
Tags: Plasma
Maniphest Tasks: T10325
Differential Revision: https://phabricator.kde.org/D19916