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/GEventLoop.cpp

@@ -158,6 +158,13 @@ int GEventLoop::exec()
             } else {
                 receiver->event(event);
             }
+
+            if (m_exit_requested) {
+                auto rejigged_event_queue = move(events);
+                rejigged_event_queue.append(move(m_queued_events));
+                m_queued_events = move(rejigged_event_queue);
+                return m_exit_code;
+            }
         }
     }
     ASSERT_NOT_REACHED();