emit Application::ready() at a suitable time during startup

This causes macOS to deliver an "openFile" event which allows us
to function correctly when started by a click on a .ardour file

gtk2_ardour/ardour_ui_startup.cc

@@ -477,10 +477,14 @@ ARDOUR_UI::sfsm_response (StartupFSM::Result r)
 int
 ARDOUR_UI::starting ()
 {
+	Application* app = Application::instance();
+
 	if (ARDOUR_COMMAND_LINE::check_announcements) {
 		check_announcements ();
 	}
 
+	app->ready ();
+
 	/* we need to create this early because it may need to set the
 	 *  audio backend end up.
 	 */