add session and power mgmt runners to kickoff

Summary:
It's a bit weird that kickoff's leave tab has power management actions but
you cannot actually invoke them via a search.

CHANGELOG: Application Launcher supports searching for power actions

Test Plan: changed, searching for shutdown lists the actual shutdown runnable as result

Reviewers: hein, broulik

Reviewed By: hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6020