LibGUI: Make GAction scoped to its CObject parent (widget or window)

Unparented GActions are still parented to the application like before,
making them globally available.

This makes it possible to have actions that work whenever a specific
window is active, no matter which widget is currently focused. :^)

Libraries/LibGUI/GWindow.h

@@ -35,6 +35,8 @@
 #include <LibDraw/Rect.h>
 #include <LibGUI/GWindowType.h>
 
+class GAction;
+class GKeyEvent;
 class GWMEvent;
 class GWidget;
 class GWindowServerConnection;
@@ -170,6 +172,8 @@ public:
 
     virtual bool is_visible_for_timer_purposes() const override { return m_visible_for_timer_purposes; }
 
+    GAction* action_for_key_event(const GKeyEvent&);
+
 protected:
     GWindow(CObject* parent = nullptr);
     virtual void wm_event(GWMEvent&);
@@ -210,3 +214,9 @@ private:
     bool m_layout_pending { false };
     bool m_visible_for_timer_purposes { true };
 };
+
+template<>
+inline bool is<GWindow>(const CObject& object)
+{
+    return object.is_window();
+}