The creation of PlasmaWindowInterface is moved from WaylandServer into
AbstractClient. This allows the sub classes to better control when to
create/destroy the Client.

For creation it's bound to becoming visible - that is Windows which are
only created but never shown are not announced at all.

For Client it's destroyed with the normal tear-down of a Client, for
ShellClient it's destroyed on unmapped (which also means a new one
will be created again in case of another mapping of the surface).

As a side effect, this works around the problem that ShellClients do not
yet get destroyed for QtWayland's menus (needs further investigation).