Constrain layout messages to be members of a Message class

Using Typeables as the only constraint on layout messages is a bit
scary, as a user can send arbitrary values to layoutMsg, whether they
make sense or not: there's basically no type feedback on the values you
supply to layoutMsg.

Folloing Simon Marlow's dynamically extensible exceptions paper, we use
an existential type, and a Message type class, to constrain valid
arguments to layoutMsg to be valid members of Message.

That is, a user writes some data type for messages their layout
algorithm accepts:

  data MyLayoutEvent = Zoom
                     | Explode
                     | Flaming3DGlassEffect
                     deriving (Typeable)

and they then add this to the set of valid message types:

  instance Message MyLayoutEvent

Done. We also reimplement the dynamic type check while we're here, to
just directly use 'cast', rather than expose a raw fromDynamic/toDyn.

With this, I'm much happier about out dynamically extensible layout
event subsystem.

Config.hs

@@ -154,8 +154,8 @@ keys = M.fromList $
     , ((modMask,               xK_j     ), raise GT)
     , ((modMask,               xK_k     ), raise LT)
 
-    , ((modMask,               xK_h     ), layoutMsg Shrink)
-    , ((modMask,               xK_l     ), layoutMsg Expand)
+    , ((modMask,               xK_h     ), sendMessage Shrink)
+    , ((modMask,               xK_l     ), sendMessage Expand)
 
     , ((modMask .|. shiftMask, xK_c     ), kill)