Add event for changes to the safe area

https://github.com/rust-windowing/winit/pull/3890 added an API to query the safe area, but left an API for listening to changes to that value unspecified (only noting that RedrawRequested would be called if it changed).

The API could look something like this:

enum WindowEvent {
   SafeAreaChanged {
        origin: PhysicalPosition<u32>,
        size: PhysicalSize<u32>,
    }
    // ...
}

But that'd mean we have to emit it whenever the surface resizes too, which isn't so nice.

Perhaps something like the following would be better?

struct Insets {
    top: PhysicalUnit<u32>,
    left: PhysicalUnit<u32>,
    bottom: PhysicalUnit<u32>,
    right: PhysicalUnit<u32>,
}

enum WindowEvent {
   SafeAreaChanged(Insets),
    // ...
}

trait Window {
    fn safe_area_insets(&self) -> Insets;
    // ...
}

See also discussion starting from https://github.com/rust-windowing/winit/pull/3890#issuecomment-2343351828.

Start with PR 3890 and the discussion linked from its issue comment to understand the existing safe-area API and the unresolved event design. Review the WindowEvent and Window API context shown in this issue, then establish a consensus on the event shape and resize behavior. Done means the API and when it is emitted are specified and implemented consistently.