Replaced the text "loading content.." by spinner loader UI

This doesn't seem to be an appropriate use of the PTBP status.

I added a template for the loading animation quicktabs-loading.html.twig so you can override this in your theme, per qt instance if needed.

$markup = [
              '#theme' => 'quicktabs_loading',
              '#message' => t('Loading content ...'),
              '#instance' => $qt_instance,
              '#tabid' => $tab_id,
            ];

Attached is a patch from !MR21 for composer.