Add delete' for use in shift

Rename delete to delete' so we can clear floating status in delete,
thus removing one special handling. 
At the moment delete' is only used in shift, but is useful for temporarily
removing a window from the stack.

StackSet.hs

@@ -26,7 +26,7 @@ module StackSet (
         focusWindow, tagMember, member, findIndex,
         -- * Modifying the stackset
         -- $modifyStackset
-        insertUp, delete, filter,
+        insertUp, delete, delete', filter,
         -- * Setting the master window
         -- $settingMW
         swapMaster, swapUp, swapDown, modify, modify', float, sink, -- needed by users
@@ -453,9 +453,14 @@ insertUp a s = if member a s then s else insert
 --   * otherwise, delete doesn't affect the master.
 --
 delete :: (Ord a, Eq s) => a -> StackSet i a s sd -> StackSet i a s sd
-delete w s = s { current = removeFromScreen (current s)
-               , visible = map removeFromScreen (visible s)
-               , hidden = map removeFromWorkspace (hidden s) }
+delete w = sink w . delete' w
+
+-- only temporarily remove the window from the stack, thereby not destroying special
+-- information saved in the Stackset
+delete' :: (Ord a, Eq s) => a -> StackSet i a s sd -> StackSet i a s sd
+delete' w s = s { current = removeFromScreen (current s)
+                , visible = map removeFromScreen (visible s)
+                , hidden = map removeFromWorkspace (hidden s) }
     where removeFromWorkspace ws = ws { stack = stack ws >>= filter (/=w) }
           removeFromScreen scr = scr { workspace = removeFromWorkspace (workspace scr) }
 
@@ -495,5 +500,5 @@ swapMaster = modify' $ \c -> case c of
 shift :: (Ord a, Eq s, Eq i) => i -> StackSet i a s sd -> StackSet i a s sd
 shift n s | n `tagMember` s && n /= curtag = maybe s go (peek s)
           | otherwise                      = s
-    where go w = view curtag . insertUp w . view n . delete w $ s
+    where go w = view curtag . insertUp w . view n . delete' w $ s
           curtag = tag (workspace (current s))