patch 7.4.1557

Problem:    Windows cannot be identified.
Solution:   Add a unique window number to each window and functions to use it.

src/structs.h

@@ -2273,6 +2273,8 @@ struct matchitem
  */
 struct window_S
 {
+    int		w_id;		    /* unique window ID */
+
     buf_T	*w_buffer;	    /* buffer we are a window into (used
 				       often, keep it the first item!) */