Add VirtualDesktops to PlasmaWindowModel

Summary:
VirtualDesktops with the new plurality was added PlasmaWindowManagement,
but PlasmaWindowModel was left unchanged.

In behavioural changes, setting a window to be on all running desktops
should not mark it as being set on all desktops, they are 2 distinct
pieces of information.

Test Plan: Relevant unit test

Reviewers: KWin, mart

Reviewed By: KWin, mart

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16883