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/InputBox.h

@@ -33,7 +33,7 @@ namespace GUI {
 class InputBox : public Dialog {
     C_OBJECT(InputBox)
 public:
-    explicit InputBox(const StringView& prompt, const StringView& title, Core::Object* parent = nullptr);
+    explicit InputBox(const StringView& prompt, const StringView& title, Window* parent_window = nullptr);
     virtual ~InputBox() override;
 
     String text_value() const { return m_text_value; }