Move lights/timers with objects that change levels

Daniel Thaler forgot to do this when he removed object migration,
leading to saved games being rendered unloadable if e.g. a light source
or rotting corpse fell down a hole or changed levels some other way.

This is meant to go with commit cc2a91866d (eliminate object migration).
Bump patchlevel to 0.5.4

This release fixes a bug where rotting corpses or light sources falling
through a hole would render a save game unloadable without manual
repair.