Merge bevy_app into bevy_ecs

`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.