Exhaustively clear input focus before unmapping

This is a fix for the following bug:
https://bbs.archlinux.org/viewtopic.php?pid=1244541#p1244541

I'm not sure why `monsterwm` is immune to this or if there's a better
way to fix it. I've made a test with a simple window listening to
focus{in,out} events and it did receive those events after being
unmapped without needing the fix...