[Window] Fix focus with metacity-2.22

[Window] Fix focus with metacity-2.22

Metacity-2.22 broke our old hack to get the focus. This hack was
suggested by the folks on GNOME's IRC channel.

I left the old _NET_WM_USER_TIME hack in place, because it does
not seem to cause any problems. It didn't seem to matter if I
removed it either, though.

Tested Window Managers:
Metacity-2.20.2
Metacity-2.22.0
KWin-3.5.9
Fluxbox-1.0.0
Openbox-3.4.7_pre3