Revert "Move window pings to MetaWindow"

This was a bad idea, as ping/pong has moved to a client-specific
request/event pair, rather than a surface-specific one. Revert
the changes we made here and correct the code to make up for it.

This reverts commit aa3643cdde.

src/core/display-private.h

@@ -55,6 +55,10 @@ typedef struct _MetaWindowPropHooks MetaWindowPropHooks;
 
 typedef struct MetaEdgeResistanceData MetaEdgeResistanceData;
 
+typedef void (* MetaWindowPingFunc) (MetaWindow  *window,
+                                     guint32      timestamp,
+                                     gpointer     user_data);
+
 typedef enum {
   META_LIST_DEFAULT                   = 0,      /* normal windows */
   META_LIST_INCLUDE_OVERRIDE_REDIRECT = 1 << 0, /* normal and O-R */
@@ -183,7 +187,7 @@ struct _MetaDisplay
   guint32 window_sequence_counter;
 
   /* Pings which we're waiting for a reply from */
-  GHashTable *pending_pings;
+  GSList     *pending_pings;
 
   /* Pending focus change */
   guint       focus_timeout_id;
@@ -449,6 +453,14 @@ void meta_display_retheme_all (void);
 void meta_display_set_cursor_theme (const char *theme, 
 				    int         size);
 
+void meta_display_ping_window      (MetaWindow         *window,
+                                    guint32             timestamp,
+                                    MetaWindowPingFunc  ping_reply_func,
+                                    MetaWindowPingFunc  ping_timeout_func,
+                                    void               *user_data);
+void meta_display_pong_for_serial  (MetaDisplay        *display,
+                                    guint32             serial);
+
 int meta_resize_gravity_from_grab_op (MetaGrabOp op);
 
 gboolean meta_grab_op_is_moving   (MetaGrabOp op);