[ksmserver] Split startup into separate class

Summary:
Everything related to Plasma startup goes in Startup everything relating
to X session management stays in KSMServer.  Restoring the window
manager currently stays inside ksmserver as it passes the session ID as
an argument and at various points checks if something the client is the
WM.
Relevant methods move to server.cpp to keep one class per cpp file.

KSMServer gains some signals as it can no longer call the startup methods
directly to continue the process.

This is part of a cleanup series in ksmserver. (The friend class will disappear
in the next commit)

Behaviour should remain identical.

Test Plan: Logged in on X

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16225