Port "Switch Window" containment action to updated libtaskmanager API and away from KWindowSystem

Summary:
This makes it work on Wayland. And probably fixes bugs, the code,
especially the next/prev window stuff, was pretty wonky.

Also some general cleanup.

Depends on D13745.

Reviewers: mart, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T4457

Differential Revision: https://phabricator.kde.org/D15642