Fix float behaviour, add shiftWin.

First, if float is called with window which is on a hidden workspace,
then the window will remain on that hidden workspace.

Now the focus should change more as expected:
float w = (view current) . (shiftWin ws w)
    where
        current is the current screen/workspace
        shiftWin ws w is: - view the workspace w is on
            - set focus on w
            - shift ws
            - set focus back to window it was on that workspace
                unless w was focused

shiftWin was add to StackSet.hs

Main.hs

@@ -29,7 +29,7 @@ import Graphics.X11.Xinerama    (getScreenInfo)
 
 import XMonad
 import Config
-import StackSet (new, floating, member, findIndex, workspace, tag, current, visible)
+import StackSet (new, floating, member)
 import qualified StackSet as W
 import Operations
 
@@ -226,10 +226,7 @@ handle e@(ConfigureRequestEvent {ev_window = w}) = withDisplay $ \dpy -> do
     ws <- gets windowset
     wa <- io $ getWindowAttributes dpy w
 
-    -- TODO temporary workaround for some bugs in float.  Don't call 'float' on
-    -- windows that aren't visible, because it changes the focused screen
-    let vis = any ((== findIndex w ws) . Just . tag . workspace) (current ws : visible ws)
-    if (M.member w (floating ws) && vis)
+    if M.member w (floating ws)
         || not (member w ws)
         then do io $ configureWindow dpy w (ev_value_mask e) $ WindowChanges
                     { wc_x            = ev_x e