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/GObject.h

@@ -33,6 +33,7 @@ public:
     void dump_tree(int indent = 0);
 
     virtual bool is_widget() const { return false; }
+    virtual bool is_window() const { return false; }
 
 protected:
     virtual void timer_event(GTimerEvent&);