LibGUI: More work on GInputBox.

- If the GInputBox has a parent and the parent is a GWindow, center the
  input box window within the parent window. This looks quite nice.

- Stop processing events in a nested event loop immediately after it's
  been asked to quit.

- Fix GWidget::parent_widget() behavior for non-widget parents.

LibGUI/GWindow.h

@@ -82,6 +82,7 @@ public:
 
 private:
     virtual const char* class_name() const override { return "GWindow"; }
+    virtual bool is_window() const override final { return true; }
 
     Retained<GraphicsBitmap> create_backing_bitmap(const Size&);
     void set_current_backing_bitmap(GraphicsBitmap&, bool flush_immediately = false);