If we use poll, then we don't need the "EPChannelCreated" signal, but that's a little CPU consuming.

If we use that signal, then we can just poll once in case a channel is created before the signal callback registered.
As you know, the app_proxy will only created after the Application->Launch() finished, the extension channel may created before we call ```g_signal_connect(app_proxy, "g-signal", G_CALLBACK(on_app_signal), NULL);```