Minor refactors/fixes (#4589)

changes:

Underwear has been promoted to a human layer; this might fix some potential issues with human icon caching.
turf/Entered() code has been cleaned up and proximity checks have been timerized.
Fixed an issue where openspaces wouldn't render objects that entered an openspace after init.
Shortened organ keymap prefix as it didn't really need to be that long.

code/modules/client/preferences.dm

@@ -387,6 +387,7 @@ datum/preferences
 		character.update_mutations(0)
 		character.update_body(0)
 		character.update_hair(0)
+		character.update_underwear(0)
 		character.update_icons()
 
 /datum/preferences/proc/open_load_dialog_sql(mob/user)