Introduce an action to display what we're showing in the menu

Summary:
We let the developer introduce a tree of actions as a menu.
This patch makes it possible for the application to see how this is being
navigated

Test Plan: tested locally

Reviewers: Kirigami, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D2831