[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