[sddm-theme/lock screen] Use default font size adjustments instead of hardcoded sizes
Summary: Same result as D19433, except not hardcoded anymore (adjusts for the default font size the user sets).
Reviewers: VDG, Plasma, filipf, ngraham
Reviewed By: VDG, filipf, ngraham
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D19547