code: public Component getLoadingComponent(final String _markupId)
      {
          final IRequestHandler handler = new ResourceReferenceRequestHandler(AbstractDefaultAjaxBehavior.INDICATOR);
          return new Label(_markupId, "<img alt=\"Loading...\" src=\"" + RequestCycle.get().urlFor(handler) + "\"/>")
                          .setEscapeModelStrings(false);
      }
docs: Gets the loading component.
  
  @param _markupId the markup id
  @return The component to show while the real component is being created.