Improve powerdevil's main

Summary:
Make it possible to restart the daemon for easier debugging.
Use Qt::AA_DisableSessionManager for all the performance.

Test Plan: Using it right now

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28492