Added an Item_manager class, and the global item_controller to reference...

... it's only instance.

It is fully initialized as part of game.cpps initialization step, and doesn't do much for the moment aside from providing another way to access the same information already accessed through the itypes map. However, it lays the groundwork for additional changes.

To insure proper functionality, calls to itypes in wish.cpp now use the new item_controller.