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