LibGUI+WindowServer: Flash menubar menu when using a keyboard shortcut

Briefly flash the menubar menu containing the keyboard shortcut action
to give the user immediate visual feedback on their interaction with the
system.

Userland/Libraries/LibGUI/Window.cpp

@@ -16,6 +16,7 @@
 #include <LibGUI/Application.h>
 #include <LibGUI/Desktop.h>
 #include <LibGUI/Event.h>
+#include <LibGUI/MenuItem.h>
 #include <LibGUI/Menubar.h>
 #include <LibGUI/Painter.h>
 #include <LibGUI/Widget.h>
@@ -1204,6 +1205,15 @@ Menu& Window::add_menu(String name)
     return *menu;
 }
 
+void Window::flash_menubar_menu_for(const MenuItem& menu_item)
+{
+    auto menu_id = menu_item.menu_id();
+    if (menu_id < 0)
+        return;
+
+    WindowServerConnection::the().async_flash_menubar_menu(m_window_id, menu_id);
+}
+
 bool Window::is_modified() const
 {
     if (!m_window_id)