Finish safe area implementation

[madsmtm]

#3890 introduced Window::safe_area, but the implementation isn't completed on all platforms. Tracking this here:

• Android
• AppKit / macOS: Implemented
• Orbital
• UIKit / iOS: Implemented
• Wayland: No protocol for that at the time of writing.
• Web: Calculate independently of scroll position, Add safe area and document coordinate systems #3890 (comment)
• Windows
• X11

Maintainers: Feel free to check the box and remove the corresponding label, either if the implementation is completed, or it doesn't apply to that platform.

Note: Make sure to also wire it up so that RedrawRequested events are emitted when the safe area / insets change.

See also #3911.

Do not close this issue before the ## Platform-specific section on safe_area is updated too.