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>

examples/file_dialog/src/main.rs

@@ -107,10 +107,10 @@ fn preview_files_being_dropped(ctx: &egui::Context) {
         let painter =
             ctx.layer_painter(LayerId::new(Order::Foreground, Id::new("file_drop_target")));
 
-        let screen_rect = ctx.screen_rect();
-        painter.rect_filled(screen_rect, 0.0, Color32::from_black_alpha(192));
+        let content_rect = ctx.content_rect();
+        painter.rect_filled(content_rect, 0.0, Color32::from_black_alpha(192));
         painter.text(
-            screen_rect.center(),
+            content_rect.center(),
             Align2::CENTER_CENTER,
             text,
             TextStyle::Heading.resolve(&ctx.style()),