LIB/GTK: use new data structure with explicit SymLink constructor

This still needs a lot of work to function consistently, but it's better
than the old approach.

src/GUI/Gtk/Callbacks.hs

@@ -111,7 +111,7 @@ urlGoTo mygui myview = do
 open :: Row -> MyGUI -> MyView -> IO ()
 open row mygui myview = withErrorDialog $
   case row of
-    r@(_ :/ Dir _ _) -> do
+    SDir r -> do
       nv <- Data.DirTree.readFile $ fullPath r
       refreshTreeView' mygui myview nv
     r ->