So far KWin resolved the logind session to take over by using its own
PID with the method GetSessionByPID. This doesn't work if KWin itself
is not part of the session or if KWin is supposed to take over a
different session.

The session to take over can now be provided by the env variable:
KWIN_LOGIN_SESSION

This allows to e.g. run kwin on vt1, but have the graphical output
on vt2. Thus it's possible to see the debug output. Also it should allow
to run kwin_wayland through an ssh session, so that gdb can be used.

If we take over another session on a different vt, we need to make sure
that it's the current virtual terminal. Otherwise assumptions in the
startup code would break. So let's ensure the session we integrate with
is active.