Adjust Task Manager to libtaskmanager API changes for Wayland virtual desktop support

Summary:
Depends on D13745.

This code is largely untested and subject to change.

Reviewers: mart, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T4457

Differential Revision: https://phabricator.kde.org/D13746