[sddm-kcm] Write settings to /etc/sddm.conf.d instead of etc/sddm.conf
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.
- 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
Differential Revision: https://phabricator.kde.org/D21832