Add a common directory for functionality being used by kcm and daemon
Certain functionality will be shared between kcm and daemon in the future,
but is for now not extensive enough to be put into libkscreen.
In order to still reduce code duplication add a common top-level directory.
Initially this directory contains the data root path logic and the Control
classed, such that the kcm can access it in the future as well.
Test Plan: Autotest passes.
Reviewers: Plasma, mart
Reviewed By: Plasma, mart
Maniphest Tasks: T10028
Differential Revision: https://phabricator.kde.org/D16993