This legacy session management scheme using the WM_COMMAND property
seems to be called XSM. It is very, very legacy because it was
superseded by XSMP in 1993(!).

By inspecting ~/.config/session/kwin_[...] I could see that Firefox
still sets WM_COMMAND, but nothing else in a regular session with
some applications.
As one of the last applications to switch to XSMP session management,
I guess Firefox keeps doing that because it still needs to work on,
say, Solaris, anyway, so why not set WM_COMMAND on Linux as well.
The WM_COMMAND set by Firefox looks like "wmCommand8=firefox\s" in
the kwin session file. It doesn't actually contain a session
identifier. But then, Firefox only has one session per user anyway.

This goes together with commit 5f0ca1305db4a925 in plasma-workspace /
ksmserver to remove legacy session management support.

I've talked about my plan to do this on IRC with Martin so hopefully
it's okay to just do this now.