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/windows/window.rs

@@ -43,7 +43,7 @@ use crate::{
         window_state::{CursorFlags, SavedWindow, WindowFlags, WindowState},
         PlatformSpecificWindowBuilderAttributes, WindowId,
     },
-    window::{CursorIcon, Fullscreen, WindowAttributes},
+    window::{CursorIcon, Fullscreen, UserAttentionType, WindowAttributes},
 };
 
 /// The Win32 implementation of the main `Window` object.
@@ -621,6 +621,37 @@ impl Window {
         warn!("`Window::set_ime_position` is ignored on Windows")
     }
 
+    #[inline]
+    pub fn request_user_attention(&self, request_type: Option<UserAttentionType>) {
+        let window = self.window.clone();
+        let active_window_handle = unsafe { winuser::GetActiveWindow() };
+        if window.0 == active_window_handle {
+            return;
+        }
+
+        self.thread_executor.execute_in_thread(move || unsafe {
+            let (flags, count) = request_type
+                .map(|ty| match ty {
+                    UserAttentionType::Critical => {
+                        (winuser::FLASHW_ALL | winuser::FLASHW_TIMERNOFG, u32::MAX)
+                    }
+                    UserAttentionType::Informational => {
+                        (winuser::FLASHW_TRAY | winuser::FLASHW_TIMERNOFG, 0)
+                    }
+                })
+                .unwrap_or((winuser::FLASHW_STOP, 0));
+
+            let mut flash_info = winuser::FLASHWINFO {
+                cbSize: mem::size_of::<winuser::FLASHWINFO>() as UINT,
+                hwnd: window.0,
+                dwFlags: flags,
+                uCount: count,
+                dwTimeout: 0,
+            };
+            winuser::FlashWindowEx(&mut flash_info);
+        });
+    }
+
     #[inline]
     pub fn is_dark_mode(&self) -> bool {
         self.window_state.lock().is_dark_mode