move custom `listeners` inside `application` sub array

We should be careful about taking over top level namespaces at this point. Could cause a lot of bc breaks for users.

We use the top level `listeners` configuration key to configure a custom event listener service locator in our internal library. I know, we probly should've prefixed it with a vendor name or placed it inside a sub array. But what's done is done I guess.

I figure since we already have `service_manager` it should be safe to take over `application` for internal use?