Sync NumLock preference

Summary:
SDDM currently doesn't respect the user's choice on whether or not NumLock should be on or off by default. Fortunately there is a config option that can be filled in with info from Plasma that only needed a little bit of translating.

FEATURE: 347290
FIXED-IN: 5.17.0

Reviewers: #plasma, ngraham, davidedmundson

Reviewed By: #plasma, ngraham, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22634