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/WindowServerConnection.cpp

@@ -172,6 +172,7 @@ void WindowServerConnection::key_down(i32 window_id, u32 code_point, u32 key, u3
 
     if (auto* action = action_for_key_event(*window, *key_event)) {
         if (action->is_enabled()) {
+            action->flash_menubar_menu();
             action->activate();
             return;
         }