Grab Pointer in TreeSelect

If this is not done, trying to select another window with the mouse when
treeselect is up, the X server may stop handling keyboard and pointer
requests, making it appear that xmonad froze.  As the pointer can't do
useful things anyways when the treeselect window is shown, simply
grabbing it solves this problem.

C.f.:
  - xmonad/xmonad/issues/116
  - xmonad/issues/445

XMonad/Actions/TreeSelect.hs

@@ -329,10 +329,11 @@ treeselectAt conf@TSConfig{..} zipper hist = withDisplay $ \display -> do
         -- and mouse button 1
         grabButton display button1 anyModifier win True buttonReleaseMask grabModeAsync grabModeAsync none none
 
-    -- grab the keyboard
-    status <- liftIO $ grabKeyboard display win True grabModeAsync grabModeAsync currentTime
+    -- grab the keyboard and the pointer
+    statusK <- liftIO $ grabKeyboard display win True grabModeAsync grabModeAsync currentTime
+    statusP <- liftIO $ grabPointer display win True none grabModeAsync grabModeAsync none none currentTime
 
-    r <- if status == grabSuccess
+    r <- if statusK == grabSuccess && statusP == grabSuccess
         then do
             -- load the XMF font
             gc <- liftIO $ createGC display win