[Xmonad] Constrained dynamically extensible layout events: a patch

[Donald Bruce Stewart]

dons:
> David, Spencer et al, are there any problems with restricting layoutMsg
> arguments to being members of the (open) Message class? I'd (strongly)
> prefer this over just Dynamic, since it restores most of the static
> checking we'd need. 

Oh, I forgot an example. This is why Dynamic-only is scary:

        , ((modMask,               xK_h     ), layoutMsg "hey Joe!")
        , ((modMask,               xK_l     ), layoutMsg Expand)

is well typed. Under the new system, it becomes a static error:

    Config.hs:157:43:
        No instance for (Message [Char])
              arising from use of `sendMessage' at Config.hs:157:43-65

-- Don