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_demo_lib/src/lib.rs

@@ -109,6 +109,6 @@ fn test_egui_zero_window_size() {
 /// Detect narrow screens. This is used to show a simpler UI on mobile devices,
 /// especially for the web demo at <https://egui.rs>.
 pub fn is_mobile(ctx: &egui::Context) -> bool {
-    let screen_size = ctx.screen_rect().size();
+    let screen_size = ctx.content_rect().size();
     screen_size.x < 550.0
 }