- Fix a bug that causes wrong focus detection result in Awesome and
maybe other window managers, when --use-ewmh-active-win is enabled and
_NET_ACTIVE_WINDOW changes before the newly-focused window is mapped.
- Fix a typo that causes more than one window to stay focused after a
window destruction with --use-ewmh-active-win.
- Fix a bug that find_clientwin() incorrectly returns a window when
window ID 0 is passed to it.
- Check for window ID 0 in update_ewmh_active_win().