[NEW] [oc-panel] Sub-Menu for Cache Menu: 1-click Cache Clear

NOTICE: don't forget to hold Shift when clicking on these 2 new links :^# (or we add a `target="_blank"` to `Theme::admin_link` and `themes/default/views/oc-panel/admin_link.php` ?

Possible Nice to Have enhancement: AJAX based instead of opening new oc-panel page