1) gimbal_type_item.h
    - Created polymorphic base type, GblTypeItem, which will serve to
      unify both the user-facing types of GblProperty and GblSignal, but
      also their back-end registry hash table structure containing
      linked lists of items associated with a given type.
    - Created vtable GblClass type which should supply polymorphic
      abstraction for all registry and derived type functionality whose
      implementation is different and can be overridden depending on
      deriving types.
    - Finalized all of public API.
    - Broke API by renaming GblProperty_foreach() to
      GblProperty_iterate(), which is in-line with the new iteration
      naming conventions we use throughout libGimbal.