wayland-server: Listen for pointer current surface destruction

Add a destroy listener so that when the current surface associated with the
pointer is destroyed we can reset the pointer to the current surface. In order
to achieve this add a wl_pointer_set_current() which handles assigning the
surface and creating the listener.

This resolves a use-after-free error triggered with nested popup surfaces

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=696946