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.

The state RestoringWMSession is dropped as it does nothing useful.

BUG: 442380


(cherry picked from commit a22ff36e)