Adjust Pager to libtaskmanager API changes for Wayland virtual desktop support

Summary:
Missing:
* The KWayland API has no way to move windows yet

Depends on D13745.

This code is largely untested and subject to change.

Reviewers: mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T4457

Differential Revision: https://phabricator.kde.org/D13748