[xmonad] Move EWMH support initialization to a startupHook

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. This complicates the user configs a bit (though they can just use Config.Desktop) but fixes problems like setWMName having to be called in the log hook if ewmh is being used.