Addressing wayland/systemd was a whole new game. I initially thought
its task is beyond the design scope of im-config. Then YOSHINO-san
proposed new hook via systemd as
/usr/lib/systemd/user-environment-generators/70-im-config .
2bbb1a3 ("70-im-config: Export env variables via systemd", 2019-03-
07)
This systemd.environment-generator(7) is invoked by `systemd --user'
(>= 233) to export variables to its activation environment, which is
used by gdm3 for environment variables in a new desktop session.
Unlike Xorg, Wayland does not use /etc/X11/Xsession.d/, where im-
config has exported environment variables. This generator should
compensate the lacking function for a Wayland desktop session
launched from gdm3.
I didn't want to create a monster which I can't mauntain, YOSHINO-san's
proposal seemed reasonably compact.