GWidget: Always update self after a child is removed.

We could probably tighten the amount of invalidation we do here, but this is
definitely more correct than not invalidating at all.

LibGUI/GWidget.cpp

@@ -35,6 +35,7 @@ void GWidget::child_event(CChildEvent& event)
             else
                 invalidate_layout();
         }
+        update();
     }
     return CObject::child_event(event);
 }