globalaccel: allow sleep and hibernate keyboard shortcuts

Summary:
This is useful for the phone specially. You can quickly turn on-off your
phone from lockscreen without actually unlocking it.

Test Plan: I need help testing it as my prefix session is broken currently.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27721