Launch Steam without tray icon and force the client to shut down after clicking on the "close window" symbol. Still very experimental.
Forked from https://github.com/dscharrer/steamwm
steamwm.cpp: will be compiled in a library that will be preloaded and sets the _NET_WM_PID value for the Steam window
wmctrl.c: modified version of wmctrl that checks if a given PID is among the open windows
hacks/libappindicator.so.1: modified appindicator library to force the creation of a Steam tray icon to fail
run-steam.sh: starts Steam and sends the "shutdown" command to it when the window disappeared