Support to set window theme and expose system window theme changed event

[lewiszlw]

Objective

• Fixes OS Dark/Light theme detection #8586.

Solution

• Add preferred_theme field to Window and set it when window creation
• Add window_theme field to InternalWindowState to store current window theme
• Expose winit WindowThemeChanged event

[mockersf]

could you add something in example https://github.com/bevyengine/bevy/blob/main/examples/window/window_settings.rs showing how to change the theme on a key press?

[hate]

Took a shot at adding the above to the example here: lewiszlw#1

[lewiszlw]

Thanks @hate to update example!

[mnmaita]

Leaving a few comments and questions. Thanks a lot for this addition!

[mnmaita]

Looks good to me. Let's wait for @mockersf blessing as he also reviewed.

[nicopap]

I think this can be merged once the few bugs, misleading doc and unused field are fixed.

[nicopap]

Suggested change

	/// - iOS / Android / Web / x11: Unsupported.
	/// Unsupported on iOS, Android, Web and X11. The return value is always the last set value or `None` on those platforms.

[nicopap]

There is a bit of an issue here.

On unsupported platforms, winit always returns None when using window.theme(). However, in this bevy implementation, it's possible to set self.internal.window_theme to Some(Theme) in set_window_theme. Now this will return a misleading value.

I suggest conditionally returning None here, something like

if cfg!(any(target_os = "ios", target_os = "android", target_family = "wasm", feature = "x11")) {
  None
} else {
  self.internal.window_theme
}

[nicopap]

Just a small change request and this looks good to me.

[nicopap]

Great! I think this is ready to merge