Also read sddm.conf.d config directories

Summary:
SDDM reads files in alphabetical order from /usr/lib/sddm/sddm.conf.d/,
then /etc/sddm.conf.d/ and /etc/sddm.conf. The latest occurence takes
precedence.
The paths in CMakeLists are copied from sddm.

Test Plan: kcmshell5 kcm_sddm now shows the right preselected values.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12359