Expose libkworkspace SessionManagement class to QML
Summary:
It was written with properties and signals intended for direct QML use.
This should be a step towards removing the plugins in the session
applets in kdeplasma-addons.
Test Plan: Used in a qml file
Reviewers: Plasma, broulik
Reviewed By: Plasma, broulik
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25048