Fix FabricIndex handling in structs and events. (#628)

1) Remove struct_is_fabric_scoped and event_is_fabric_scoped now that we have
   isFabricScoped/isFabricSensitive on those objects.  Add the relevant XML
   attributes instead of trying to guess based on fields.
2) Remove event_fabric_idx_field and struct_fabric_idx_field, since we are no
   longer guessing based on fields.
3) Synthesize the relevant struct item or event field with the right name, type,
   field id, per spec, if the struct is fabric-scoped or the event is
   fabric-sensitive.

This is a hard requirement for implementing Leave events properly for Matter:
those have a fabric_idx field but are _not_ fabric-scoped.