Redesign Advanced tab

Summary: This patch redesigns and modernizes the Advanced tab of SDDM's KCM.

Test Plan:
Before:

After:

Entries were still being written to /etc/sddm.conf

Reviewers: ngraham, VDG, davidedmundson

Reviewed By: ngraham, VDG

Subscribers: abetts, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21610