WindowServer+LibGUI: Add support for nested menus

It's now possible to add a GMenu as a submenu of another GMenu.
Simply use the GMenu::add_submenu(NonnullOwnPtr<GMenu>) API :^)

The WindowServer now keeps track of a stack of open menus rather than
just one "current menu". This code needs a bit more work, but the basic
functionality is now here!

Servers/WindowServer/WSMenuItem.cpp

@@ -1,4 +1,5 @@
 #include "WSMenuItem.h"
+#include "WSClientConnection.h"
 #include "WSMenu.h"
 #include <LibDraw/GraphicsBitmap.h>
 
@@ -40,3 +41,10 @@ void WSMenuItem::set_checked(bool checked)
     m_checked = checked;
     m_menu.redraw();
 }
+
+WSMenu* WSMenuItem::submenu()
+{
+    ASSERT(is_submenu());
+    ASSERT(m_menu.client());
+    return m_menu.client()->find_menu_by_id(m_submenu_id);
+}