bevy_app is a thin layer over bevy_ecs, and is nearly impossible to use on its own.
Moreover, most standalone uses of bevy_ecs would be better suited by using the tools provided with bevy_app; particularly if we had better documentation around custom runners and manually controlling the App loop.

For example: bevy_app re-exports all of bevy_ecs::event.

This is pointless, because bevy_ecs is an explicit dependency of all users of bevy_app.
Furthermore, it creates confusion in the main Bevy docs, since the bevy_app version is listed.

This re-export can and should just be removed.