media-session: improve cleanup of objects

Don't try to use the proxy destroy event to destroy the objects.
It is not called automatically anymore, only when we call
pw_proxy_destroy() ourselves.

Destroy the proxy objects when we destroy the session managed
object instead, either when the global is removed or when we
explicitly call _destroy()

Add an object free signal to clean up final resources after the
proxies have been destroyed, like closing libraries.

Track and destroy our link proxies.

src/examples/media-session/media-session.h

@@ -43,6 +43,7 @@ struct sm_object_events {
 
 	void (*update) (void *data);
 	void (*destroy) (void *data);
+	void (*free) (void *data);
 };
 
 struct sm_object_methods {