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).
(cherry picked from commit 90f16c9c5999d7898cfe69e32e488bb68bf1ad2b)
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.
(cherry picked from commit 7d3076e70ccb1655896fdffd3e3cdeb3d882619b)