WindowServer: Add support for default MenuItem

This allows marking a MenuItem as a default action, e.g. in a
context menu for an action that reflects what e.g. a double click
would perform.

Also enhance the window menu to mark the close action as the
default, and when double clicked perform that action.

Fixes #1289

Services/WindowServer/Menu.h

@@ -87,6 +87,8 @@ public:
     bool is_window_menu_open() { return m_is_window_menu_open; }
     void set_window_menu_open(bool is_open) { m_is_window_menu_open = is_open; }
 
+    bool activate_default();
+
     int content_width() const;
 
     int item_height() const { return 20; }