LibGUI+WindowServer: Add app-global keyboard shortcuts.

This patch adds a GShortcut class. Each GAction can have a GShortcut which
will cause the event loop to listen for that key combination app-globally
and activate the event in case it's pressed.

The shortcut will also be displayed when the action is added to a menu.

Use this to hook up Alt+Up with the "open parent directory" action in the
FileManager app. :^)

WindowServer/WSMenu.h

@@ -73,6 +73,7 @@ public:
     void close();
 
 private:
+    int padding_between_text_and_shortcut() const { return 50; }
     void did_activate(WSMenuItem&);
     WSClientConnection* m_client { nullptr };
     int m_menu_id { 0 };