screen-cast-stream: Don't broadcast PipeWireStreamAdded signal

The helper function from gdbus-codegen broadcasts the signal emission,
but we really only care about sending it to the specific peer that
created the session. Thus, only emit the signal to the particular peer
that owns the session.

https://bugzilla.gnome.org/show_bug.cgi?id=784199

src/backends/meta-screen-cast-session.c

@@ -173,6 +173,12 @@ meta_screen_cast_session_get_object_path (MetaScreenCastSession *session)
   return session->object_path;
 }
 
+char *
+meta_screen_cast_session_get_peer_name (MetaScreenCastSession *session)
+{
+  return session->peer_name;
+}
+
 static gboolean
 check_permission (MetaScreenCastSession *session,
                   GDBusMethodInvocation *invocation)