Handle Ctrl+C in the terminal properly

[SarthakSingh31]

Objective

Fixes #13995.

Solution

Override the default Ctrl+C handler with one that sends AppExit event to every app with TerminalCtrlCHandlerPlugin.

Testing

Tested by running the 3d_scene example and hitting Ctrl+C in the terminal.

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Changelog

Handles Ctrl+C in the terminal gracefully.

Migration Guide

If you are overriding the Ctrl+C handler then you should call TerminalCtrlCHandlerPlugin::gracefully_exit from your handler. It will tell the app to exit.

[JMS55]

Is there a reason this is public? I'd rather hide it if not.

[janhohenheim]

See their migration guide. If I understand it correctly, the idea is that you can still call this manually to notify all apps even when you're doing your own, manual CTRL-C handling.

[SarthakSingh31]

I renamed it but the idea was that if a user defines their own ctrlc handler then they will be able to use the plugins exit function to in that handler to exit bevy. They would not have to setup their own system and static flag.

[mockersf]

could you use try_set_handler instead, and handle errors to log if there's already one set instead of panicking?

[SarthakSingh31]

set_handler always overwrites the current handler so it should not panic.
But I am changing it to try_set_handle so that the user can set their own handler. They can then call our TerminalCtrlCHandlerPlugin::gracefully_exit in their handler to exit bevy gracefully.

[mockersf]

should this be behind a feature?

[JMS55]

Imo, no. We already have a lot of features and have issues testing every combination. Users can just disable the plugin if they don't want it. I guess they'd still have a dependency on ctrlc though, but eh.

[janhohenheim]

Looks mostly good to me, left some minor notes. I don't like that fact that the graceful shutdown is broadcast globally, but it shouldn't matter in practice.

[janhohenheim]

One last rephrase, then I'm good :)

[janhohenheim]

LGTM now :)
FYI, the PR is squashed anyways, so you don't need to force push your changes during a review to keep the history clean.