Problem: After installation picom did not start because it lacked a config
file.

lxqt-config starts compton-conf which writes to .config/compton.conf but picom
is used as compositor and expects to read from .config/picom.conf

Further: on manual startup picom complains about deprecated config entries that
got written by the currently used tool.

What is the solution? For now I just softlinked picom.conf to compton.conf, but
it looks like some clean fix is needed and to make it work out of the box.