Fix session restore + kwin interaction race
KSMServer sets which session to use on startup
KSmServer::restoreSession(QString)
When the session is up and ready either systemd or plasma-session invoke
restoreSession() that actively starts restoring clients.
At some point we need to notify kwin which session we're using so that it can handle restorng window properties such as the virtual desktop.
This was done in the first call, but this has no guarantee that kwin has started yet. Without kwin getting the DBus call it has no information to match up restoration information.