Redesign Advanced tab

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

Test Plan:
Before:
{F6872046}

After:
{F6872421}

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