make workspaceDir work even in workspaces with no windows.
make workspaceDir work even in workspaces with no windows.

This also fixes a (minor) bug when the focussed window is present on
multiple visible workspaces.

darcs-hash:20080326152708-72aca-d34a7d717c984a9bdde64f1f63d334f0cb38b30f

kmels/xmonad-launcher,kmels/xmonad-launcher