Fix some logic with recording a level's XP.

Previously, we tried to do this during the `PLAYER_LEVEL_UP` event, but
the current level is not well defined during this event. Now we use a
timer to do it two seconds after leveling up.
Add option to toggle extra level logging.

Previously, we would record the current level every time the player
gained XP. This is a lot of unnecessary data, and a few levels' worth of
data indicates that it is not necessary, as our mechanism of storing and
backtracking to the last level-up event works fine.

Rather than remove the code altogether, I've added a new configuration
option to control it. (This is an excuse to try out the config registry
as much as it is useful functionality.)