[xmonad] run a program when switching to a workspace

[Wirt Wolff]

Just to make sure it makes sense -- the log hook runs on all state
changes -- focus, layout change... This might be why first suggestion
was to add to keybinding that switches to that workspace.

If you use many ways to change to that special workspace, like CycleWS
and fancy clickable dzen areas, you could make a top level function
'toSpecialWS' using the '... >> ...' code from earlier, and re-use that
function all the places you need it.

-- 
wmw