Split ksmserver into a small application that handles startup and ksmserver
This allows new startup procedures to use a session manager and also
allows us to slowly replace the session manager without breaking plasma.
Following on from the previous refactor, Startup and Shutdown are moved
from kmserver and the previous calls into KSMserver are replaced with
DBus calls.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D21995