Allow unfocused window creation (#19237)

# Objective

Allow creating a new window without it being focused, when `Window`'s
`focused` is `false.
## Solution

Use `winit`'s `WindowBuilder`'s `with_active` method

## Notes

- `winit`'s doc lists [redox's
`Orbital`](https://gitlab.redox-os.org/redox-os/orbital) as an
unsupported platform, but since Bevy doesn't officially support this
platform, I didn't put it in the documentation.

- I only tested on Linux, which is an unsupported platform. I can give
you a test code if you want to test on another platform.

- I initially put a line
[here](https://github.com/bevyengine/bevy/blob/v0.11.0/crates/bevy_winit/src/system.rs#L72)
to set the Bevy `Window`'s `focused` to `winit_window.has_focus()` after
window creation to avoid the case where `with_active` is not supported,
the window is spawned focused, no `WindowFocused` event is triggered,
and Bevy `Window` would be desynced from winit's window. But after
testing on Linux (which doesn't support `with_active`) it seems like at
that point `has_focus` returns `false` and the event is triggered, so I
removed it. Do you think I should add it back to be safe?

## Changelog

- A new unfocused `Window` can be created by setting `focused` to
`false`.

## Migration Guide

- If a `Window` is spawned with `focused` set to `false`, it will now
start not focused on supported platforms.

Adopted from #9208

---------

Co-authored-by: Sélène Amanita <selene.amanita@net-c.com>
Co-authored-by: Sélène Amanita <134181069+Selene-Amanita@users.noreply.github.com>
Co-authored-by: atlv <email@atlasdostal.com>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_window/src/window.rs

@@ -225,6 +225,15 @@ pub struct Window {
     /// You should also set the window `composite_alpha_mode` to `CompositeAlphaMode::PostMultiplied`.
     pub transparent: bool,
     /// Get/set whether the window is focused.
+    ///
+    /// It cannot be set unfocused after creation.
+    ///
+    /// ## Platform-specific
+    ///
+    /// - iOS / Android / X11 / Wayland: Spawning unfocused is
+    ///   [not supported](https://docs.rs/winit/latest/winit/window/struct.WindowAttributes.html#method.with_active).
+    /// - iOS / Android / Web / Wayland: Setting focused after creation is
+    ///   [not supported](https://docs.rs/winit/latest/winit/window/struct.Window.html#method.focus_window).
     pub focused: bool,
     /// Where should the window appear relative to other overlapping window.
     ///

crates/bevy_winit/src/winit_windows.rs

@@ -129,7 +129,8 @@ impl WinitWindows {
             .with_resizable(window.resizable)
             .with_enabled_buttons(convert_enabled_buttons(window.enabled_buttons))
             .with_decorations(window.decorations)
-            .with_transparent(window.transparent);
+            .with_transparent(window.transparent)
+            .with_active(window.focused);
 
         #[cfg(target_os = "windows")]
         {