"dead" objects removed from list

-Objects with a variable 'alive' set to false are now removed from the
'objects' list and their object type specific list, after all the step()
functions have been run.

-Made objectTypes extract the name of their ingame constructor and store
it in a variable rather than having the extraction code wherever the
name was needed.

-Game objects that are loaded from a level now store the index of their
objectType in a variable objectTypeInd.