Add support for the safe area on iOS (#7578)

This PR is a continuation of #4915 by @frederik-uni and @lucasmerlin
that introduces support for keeping egui content within the 'safe area'
on iOS (avoiding the notch / dynamic island / menu bar etc.), with the
following changes:

- `SafeArea` now wraps `MarginF32` and has been renamed to
`SafeAreaInsets` to clarify its purpose.
- `InputState::screen_rect` is now marked as deprecated in favour of
either `viewport_rect` (which contains the entire screen), or
`content_rect` (which is the viewport rect with the safe area insets
removed).
- I added some comments to the safe area insets logic pointing out the
[safe area API coming in winit
v0.31](https://github.com/rust-windowing/winit/issues/3910).

---------

Co-authored-by: frederik-uni <147479464+frederik-uni@users.noreply.github.com>
Co-authored-by: Lucas Meurer <hi@lucasmerlin.me>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui-winit/Cargo.toml

@@ -76,6 +76,24 @@ document-features = { workspace = true, optional = true }
 serde = { workspace = true, optional = true }
 webbrowser = { version = "1.0.0", optional = true }
 
+[target.'cfg(target_os = "ios")'.dependencies]
+objc2 = { workspace = true }
+objc2-foundation = { workspace = true, features = [
+  "std",
+  "NSThread",
+] }
+objc2-ui-kit = { workspace = true, features = [
+  "std",
+  "UIApplication",
+  "UIGeometry",
+  "UIResponder",
+  "UIScene",
+  "UISceneDefinitions",
+  "UIView",
+  "UIWindow",
+  "UIWindowScene",
+] }
+
 [target.'cfg(any(target_os="linux", target_os="dragonfly", target_os="freebsd", target_os="netbsd", target_os="openbsd"))'.dependencies]
 smithay-clipboard = { version = "0.7.2", optional = true }