LibGUI: Make GWindow drive relayout and do it recursively

Instead of only doing a relayout in the widget you're invalidating,
we now do a recursive top-down relayout so everything gets updated.

This fixes invalid results after updating a preferred size in some
situations with nested layouts.

Libraries/LibGUI/GWindow.h

@@ -131,6 +131,8 @@ public:
 
     virtual void save_to(AK::JsonObject&) override;
 
+    void schedule_relayout();
+
 protected:
     GWindow(CObject* parent = nullptr);
     virtual void wm_event(GWMEvent&);
@@ -175,4 +177,5 @@ private:
     String m_entered_keybind;
     int m_max_keybind_length { 0 };
     HashMap<String, WeakPtr<GWidget>> m_keyboard_activation_targets;
+    bool m_layout_pending { false };
 };