[wayland] Destroy XdgToplevelClient and XdgPopupClient on unmap

There are several ways to handle unmapping of a wl_surface. The first
one is to destroy the associated AbstractClient instance. The second one
is to transition the AbstractClient in a special state.

The problem with the second approach is that it makes animations such as
fade out more difficult to handle since effects in kwin are geared more
towards the first approach (destroying AbstractClient).