Add new signal for default action activation

Summary: activated is an overloaded signal. Add a new signal to replace the argument-less overload.

Reviewers: Frameworks, broulik, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26771