wayland/actor-surface: Always store away frame callbacks on commit

We're expected by MetaWaylandSurface to always pick the frame callbacks out from the pending state when committing (applying) so that no frame callbacks are unaccounted for. We failed to do this if our actor for some reason (e.g. associated window was unmanaged) was destroyed. To handle this situation better, store away the frame callbacks until we some later point in time need to pass them on forward.