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