[sddm-kcm] Write settings to /etc/sddm.conf.d instead of etc/sddm.conf

Summary:
As of SDDM release 0.16.0 it advised to use /etc/sddm.conf.d/ as the directory for user settings.

This patch aims to achieve it by saving in that very same directory under the filename "kde_settings.conf".

It must also remove identical entries from the old "sddm.conf" file because SDDM will simply load them instead of new user set values.

BUG: 386241
FIXED-IN: 5.17

Test Plan:
- compiles
- writes to new config file
- reads from new config file
- deletes identical entries from old config file when writing them to new one

Reviewers: davidedmundson, ngraham, #plasma

Reviewed By: davidedmundson, ngraham, #plasma

Subscribers: GB_2, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21832