wayland/pointer: Use glib signals tracking focus surface

Use the "destroy" MetaWaylandSurface signal instead of the wl_resource
destroy signal for tracking the lifetime of the surface with pointer
focus.

As unsetting the focus may have side effects due to handlers of the
"focus-surface-changed" signal, connect the signal after the default
handler to make sure other clean up facilities have the chance deal with
the surface destruction before we try to unset the focus.

https://bugzilla.gnome.org/show_bug.cgi?id=783113