LibGUI: Don't use Core::Object::add() to instantiate dialogs

Now that add() returns a WidgetType&, we can't rely on the parent of a
GUI::Dialog to still keep it alive after exec() returns. This happens
because exec() will call remove_from_parent() on itself before
returning.

And so we go back to the old idiom for creating a GUI::Dialog centered
above a specific window. Just call GUI::Dialog::construct(), passing
the "parent" window as the last parameter.

Libraries/LibGUI/Dialog.h

@@ -53,7 +53,7 @@ public:
     virtual void close() override;
 
 protected:
-    explicit Dialog(Core::Object* parent);
+    explicit Dialog(Window* parent_window);
 
 private:
     OwnPtr<Core::EventLoop> m_event_loop;