This seems a bit "too powerful" and unlikely to be used by most
applications. Remove it from now, until someone comes up with a strong
desire for it.
https://bugzilla.gnome.org/show_bug.cgi?id=670485
We don't expose ::quit-requested as API anymore. Instead, we expect
users to register inhibitors when needed. Without quit-requested,
there is no need for ::quit-cancelled and gtk_application_quit_response
anymore.
We still emit ::quit when the application is about to quit.
