Fix 'Loading...' action not showing

Summary: When the 'Share' button is clicked and the Kipi plugins are not loaded yet the menu is supposed to show {nav Loading...} but D13289 disabled that behaviour.

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Differential Revision: https://phabricator.kde.org/D13312