wayland: Rework how surface destruction works

To prevent the MetaSurfaceActor from being destroyed, we normally
unparent it before we unmanage the window. However, this doesn't
work for XWayland windows, which we unmanage when we get UnmapNotify
or DestroyNotify, not when we get the wl_surface_destroy.

To solve this, add an early hook in meta_window_unmanage that
unparents the surface actor if we have one. At the same time, clean
up the destruction code to remove old comments and assumptions about
how wl_shell behaves.