Port 'Add To Addressbook' action to Akonadi::TagWidget

Summary:
They both look and behave the same, but Akonadi::TagWidget is the
"standard" widget used everywhere else.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29252