[kcms/ksmserver] Port to new kworkspace API

Summary: requestShutDown is replaced

Test Plan:
Bodged the code so that it made my system think it had firmware support
Clicked checkbox, pressed button. Got my shutdown prompt

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21763