Change semantics of 'promote'.

Previously 'promote' would move the currently focused window into the
master position in tiled mode. This was *almost* a cycle of the windows,
but not quite (depending on where the focus was, it was in fact a
cycle).

Now we do the obvious generalisation, and just cycle the current window
stack. Simpler to understand, simpler to reason about.

Config.hs

@@ -123,7 +123,7 @@ keys = M.fromList $
     , ((modMask .|. shiftMask, xK_q                     ), io $ exitWith ExitSuccess)
     , ((modMask .|. shiftMask .|. controlMask, xK_q     ), io restart)
 
-    -- Move focused window into master position in tiling mode.
+    -- Cycle the current tiling order
     , ((modMask,               xK_Return), promote)
 
     ] ++