This is actually a followup to another task (#191).

We have environment variables for the current window (CLIENT_PID & CLIENT_WINDOW), but what about the previously focused windows? What if I'd like to run a script that does work on both the old and new window values?

One possibility is to simply work with temporary files to store the values as they get passed, but I think it should also be feasible that PekWM could include environmental variables for the previous window as well.

Thoughts?