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