Add 'request_user_attention' to Window

This commit introduces a cross platform way to request a user attention
to the window via a 'request_user_attention' method on a Window struct.
This method is inspired by macOS's 'request_user_attention' method and
thus reuses its signature and semantics to some extent.

src/platform_impl/ios/window.rs

@@ -22,7 +22,9 @@ use crate::{
         },
         monitor, view, EventLoopWindowTarget, MonitorHandle,
     },
-    window::{CursorIcon, Fullscreen, WindowAttributes, WindowId as RootWindowId},
+    window::{
+        CursorIcon, Fullscreen, UserAttentionType, WindowAttributes, WindowId as RootWindowId,
+    },
 };
 
 pub struct Inner {
@@ -260,6 +262,10 @@ impl Inner {
         warn!("`Window::set_ime_position` is ignored on iOS")
     }
 
+    pub fn request_user_attention(&self, _request_type: Option<UserAttentionType>) {
+        warn!("`Window::request_user_attention` is ignored on iOS")
+    }
+
     // Allow directly accessing the current monitor internally without unwrapping.
     fn current_monitor_inner(&self) -> RootMonitorHandle {
         unsafe {