Summary:
The inhibition was dysfunctional, as powerdevil delays the inhibition
by 5 seconds (and is already stopped when the delay times out). Also
powerdevil handles concurrent user session shutdowns and sleep requests
itself, so the code is obsolete.

As KDELibs4Support pulled in a number of Frameworks, we have to
explicitly add these now.

Reviewers: #plasma, broulik, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: apol, sitter, anthonyfieroni, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22296