Support for developing MacOS Screensavers

## What problem does this solve or what need does it fill?

Bevy would be an excellent platform to build screensavers for MacOS.

Supporting screensavers requires building a special bundle. Building bundles containing Bevy apps is commonly done. However this API requires that the executable must expose a special Objective-C class. That class has a few hooks and the OS effectively runs its own event loop.

## What solution would you like?

I'd like a way to implement this Objective-C API in a Bevy executable. Or, if that's not feasible, build a Bevy app as a library that can be called from an Objective-C stub executable.

## What alternative(s) have you considered?

I've looked around and can't find anything. Unfortunately, the Screensaver API does not let the application make its own window. This rules out most simple options I could think of.

## Additional context

The Screensaver API can be found [here](https://developer.apple.com/documentation/screensaver).