Allow users to close one window in a multi-window app

[alice-i-cecile]

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

The entire app is shut down if any window is closed.

What solution would you like?

Allow users to close one window at a time, building off of #2898.

[alice-i-cecile]

This exiting behavior can be configured https://github.com/bevyengine/bevy/blob/0bf90bb98db0b977a35d4fb39c06e9dca9bf7f04/crates/bevy_window/src/lib.rs and handled manually.

The configuration should likely be changed to use resources-as-config, and add some built-in preset to handle several common cases: "exit when the last window is closed" (likely the default), "exit when the main window is closed" and "do not exit when windows are closed". The current "exit when any window is closed" behavior doesn't have any compelling use cases IMO.

These systems should hook into the existing CloseWindow event.

For reference, this is the existing window exit system:

pub fn exit_on_window_close_system(
    mut app_exit_events: EventWriter<AppExit>,
    mut window_close_requested_events: EventReader<WindowCloseRequested>,
) {
    if window_close_requested_events.iter().next().is_some() {
        app_exit_events.send(AppExit);
    }
}