Implement support for virtual desktops on Wayland

Summary:
Includes, among other things:
* A refactoring towards supporting more than one desktop per window,
  for an eventual virtual desktops / activities merge
* A scheme for process-internal window ids on Wayland as DND payload
  so DND in and to the Pager works in the shell
* Implemented various previously missing behavior in WaylandTasksModel
  such as implicit moves of windows to the current desktop on various
  actions
* Expanded VirtualDesktopInfo API so the Pager can better abstract
  over windowing systems
* Implicit internal sharing of VirtualDesktopInfo since there are many
  more instances now
* Various cleanups

Still missing:
* Fixing the VirtualDesktops data role in the grouping proxy
* The protocol doesn't have desktop creation/destruction yet, so some
  of the related logic is still missing
* Some FIXME TODOs in the code when I was unhappy with the current
  KWayland API

This code is largely untested and subject to change.

Depends on D12820 and relates to T4457.

Reviewers: mart, mvourlakos, davidedmundson

Reviewed By: davidedmundson

Subscribers: alexde, anthonyfieroni, zzag, ngraham, abetts, plasma-devel

Tags: #plasma

Maniphest Tasks: T4457

Differential Revision: https://phabricator.kde.org/D13745