implement virtual desktop support for Wayland.
use the new virtual desktop protocol from D12820
The VirtualDesktopManager class needed some big change in order
to accomodate it, which is where most changes are.
Other than that, it's mostly connections to wire up
VirtualDesktopsManager and VirtualDesktopsManagement(the wayland protocol impl)
Other notable detail, is the client visibility updated to reflect the presence
of the client in the plasmavirtualdesktop.
(and the unSetDesktop concept)