WindowServer: Start implementing a menu system.

I'm going with a global top-of-the-screen menu instead of per-window menus.
The basic idea is that menus will live in the WindowServer and clients can
create menus via WindowServer requests.

WindowServer/WSMenuBar.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "WSMenu.h"
+#include <AK/Vector.h>
+
+class WSMenuBar {
+public:
+    WSMenuBar();
+    ~WSMenuBar();
+
+    void add_menu(OwnPtr<WSMenu>&& menu) { m_menus.append(move(menu)); }
+
+    template<typename Callback>
+    void for_each_menu(Callback callback)
+    {
+        for (auto& menu : m_menus) {
+            if (!callback(*menu))
+                return;
+        }
+    }
+
+private:
+    Vector<OwnPtr<WSMenu>> m_menus;
+};