Rework Item_factory to use generic_factory

[KorGgenT]

Is your feature request related to a problem? Please describe.
Item_factory is a specialized class that was written before the implementation of generic_factory. As such, it does not take advantage of some of the newer features of that. In addition, it is a little bit more difficult to see what's going on since it doesn't follow the same implementation as much of the rest of our json loading.
Describe the solution you'd like
Reimplement Item_factory as a generic_factory. There is plenty of documentation on generic_factory in generic_factory.h, it is quite straightforward to use and it does not necessarily require understanding on the underlying template functions.