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.

src/wayland/meta-wayland-surface.h

@@ -118,6 +118,9 @@ MetaWaylandSurface *meta_wayland_surface_create (MetaWaylandCompositor *composit
 						 struct wl_client      *client,
 						 guint32                id,
 						 guint32                version);
+
+void                meta_wayland_surface_window_unmanaged (MetaWaylandSurface *surface);
+
 void                meta_wayland_surface_configure_notify (MetaWaylandSurface *surface,
 							   int                 width,
 							   int                 height);