first shot at a floating layer

This is a first attempting at a floating layer:

mod-button1: move window
mod-button2: swapMaster
mod-button3: resize window

mod-t: make floating window tiled again

Moving or resizing a window automatically makes it floating.

Known issues:

Hard to manage stacking order. You can promote a window to move it to the top,
(which you can do with mod-button2) but it should be easier than that.

Moving a window by dragging it to a different Xinerama screen does not move it
to that workspace.

Code is ugly.

Config.hs

@@ -115,6 +115,8 @@ keys = M.fromList $
     , ((modMask,               xK_h     ), sendMessage Shrink) -- @@ Shrink the master area
     , ((modMask,               xK_l     ), sendMessage Expand) -- @@ Expand the master area
 
+    , ((modMask,               xK_t     ), withFocused clearFloating) -- @@ Make floating window tiled
+
     -- increase or decrease number of windows in the master area
     , ((modMask              , xK_comma ), sendMessage (IncMasterN 1)) -- @@ Increment the number of windows in the master area
     , ((modMask              , xK_period), sendMessage (IncMasterN (-1))) -- @@ Deincrement the number of windows in the master area