Plumb menu item activation events from WindowServer to clients.

GMenu now has an "on_item_activation" callback that fires whenever one
of its items are activated. The menu item identifier is used to distinguish
between items.

Use this to implement font switching in Terminal. :^)

LibGUI/GMenu.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <LibGUI/GMenuItem.h>
+#include <AK/Function.h>
 #include <AK/Vector.h>
 
 class GMenu {
@@ -8,9 +9,13 @@ public:
     explicit GMenu(const String& name);
     ~GMenu();
 
+    static GMenu* from_menu_id(int);
+
     void add_item(unsigned identifier, const String& text);
     void add_separator();
 
+    Function<void(unsigned)> on_item_activation;
+
 private:
     friend class GMenuBar;
     int menu_id() const { return m_menu_id; }