flush ButtonInput<KeyCode> cache when Bevy window loses focus

[gavlig]

Objective

This helps avoiding stuck key presses after switching from and back to Bevy window. Key press event gets stuck because window loses focus before receiving a key release event thus we end up with false positive in ButtonInput.

Solution

I saw two ways to fix this:

1. add bevy_window as dependency and read WindowFocus events
2. add a KeyboardFocusLost event specifically for this.

I chose the latter because adding another dependency felt wrong, but if that is more preferable changing this pr won't be a problem. Also if someone sees another way please let me know.

---

To test the bug use this small modification over examples/keyboard_input.rs:
(it will work only if you have Alt-Tab combination for switching between windows in your OS, otherwise change AltLeft accordingly)

//! Demonstrates handling a key press/release.

use bevy::{prelude::*, input::keyboard::KeyboardInput};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Update, keyboard_input_system)
        .run();
}

/// This system prints 'Alt' key state
fn keyboard_input_system(keyboard_input: Res<ButtonInput<KeyCode>>, mut keyboard_input_events: EventReader<KeyboardInput>) {
    for event in keyboard_input_events.read() {
        info!("{:?}", event);
    }

    if keyboard_input.pressed(KeyCode::AltLeft) {
        info!("'Alt' currently pressed");
    }

    if keyboard_input.just_pressed(KeyCode::AltLeft) {
        info!("'Alt' just pressed");
    }
    if keyboard_input.just_released(KeyCode::AltLeft) {
        info!("'Alt' just released");
    }
}

Here i made a quick video with demo of the fix: https://youtu.be/qTvUCk4IHvo
In first part i press Alt and Alt+Tab to switch back and forth from example app, logs will indicate that too. In second part I applied fix and you'll see that Alt will no longer be pressed when window gets unfocused

Migration Guide

WinitEvent has a new enum variant: WinitEvent::KeyboardFocusLost.

[github-actions]

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[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[alice-i-cecile]

I've run into this bug before and found it quite frustrating. I think this is a good fix that should result in consistent cross-platform behavior.