Shutdown and cleanup missing behavior

Currently, when you cleanly quit the game, there is no clean-up actions that can be ran (disconnecting from a server, saving something to a file, etc...)

There should be a way that the engine call the code from the user when the game is shutting down.

There is a method in Application::shutdown which is unused and could be re-used for that purpose.

There are two places (or more?) where exit code could be placed: State and System.

State is annoying, because you will probably end up copy pasting the shutdown method between your states.

System on the other hand would be a bit better since you could have a shutdown() method added to them. However, usually you will want your systems to be stateless, so having a cleanup method isn't really idea here either.

An rfc would need to be opened for discussion by the person taking this issue in charge.
Thanks!