[xmonad] Move EWMH support initialization to a startupHook

[Wirt Wolff]

Excerpts from Justin Bogner's message of Sat Oct 10 23:44:51 -0600 2009:
> This patch will break existing configs.
> 
> EwhmDesktops.setSupported sets some X atoms to constants. This should
> rightfully be done once, in a startup hook, rather than repeatedly in
> the log hook.

Another point in favor of applying the change pre-0.9 is that non-darcs
users that explicitly use EwmhDesktops rather than one of the desktop
configs already must change layoutHook and add handleEventHook when
upgrading to 0.9. Modifying startupHook at the same time is not much
more to ask.
-- 
wmw