Add an alternative winit runner that can be started when not on the main thread

[refnil]

Add a variant of the function winit_runner.
That variant uses EventLoop::new_any_thread instead of the regular EventLoop::new that panic if not executed on the main thread.

My use case is running bevy in a test suite. See #1057 for details.

If you prefer, I could redo this functionality using configuration in WinitConfig. I would add a field to configure if you want the regular event loop, the any thread event loop or a custom event loop.

[cart]

Looks good to me. I'm also down for moving this to WinitConfig, but I'm cool with the current approach as well.