Add KWayland virtual desktop protocol

Summary:
Implement the virtual desktop protocol discussed in
T4457 xml protocol, client and server part.

The PlasmaVirtualDesktopManagement interface manages the desktops
instantiation and layout, each desktop is a PlasmaVirtualDesktop
instance which contains unique id, name and position.

PlasmaWindow has new events: plasmaVirtualDesktopEntered
and plasmaVirtualDesktopLeft when a window enters or leaves a desktop,
and desktops as the list of desktops is in. A window can be on
any subset of desktops, if the list is empty, it's considered on all desktops.

Test Plan: Autotest

Reviewers: KWin, Plasma, graesslin, hein, davidedmundson

Reviewed By: KWin, Plasma, davidedmundson

Subscribers: davidedmundson, zzag, bshah, romangg, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T4457

Differential Revision: https://phabricator.kde.org/D12820