Add display event to acknowledge ID deletion

We need to make sure the client doesn't reuse an object ID until the
server has seen the destroy request.  When a client destroys an ID
the server will now respond with the display.delete_id event, which lets
the client block reuse until it receives the event.

src/wayland-util.h

@@ -44,6 +44,8 @@ extern "C" {
 	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
 	(type *)( (char *)__mptr - offsetof(type,member) );})
 
+#define WL_ZOMBIE_OBJECT ((void *) 2)
+
 struct wl_message {
 	const char *name;
 	const char *signature;