1. implement protocol message SendHeldItemChange
2. add save / load inventory equipped item slot in JSON
3. send held item slot message after player connect to server
Fixes#4189
* Fixes a number of "<function>: not all control paths return a value" warnings on MSVC.
* Introduces the UNREACHABLE global macro and uses it instead of conditionally compiled switch defaults.
* Move cNBTParseErrorCategory from FastNBT.h into FastNBT.cpp to prevent bad calls to message()
* Added GetPacketId method to protocol, implemented for all protocols.
* Moved GetPacketID methods into a single file, alpha-sorted.
* Fixed 1.12.1 HandlePacket switch statement.
* Added SendLogin to the GetPacketId framework.
* Added SpawnObject to GetPacketId framework.
* Added missing sendEntityEquipment packet ID update for 1.12.1
* Added LeashEntity packet ID change to 1.12.1
* Alphabetized packet enum, added SpawnGlobalEntity to GetPacketId framework
* Fixed clang errors
* Indented cases, expanded comment for GetPacketId
* Changed dyslexic comment.
changyong guo
peterbell10
Bond-009
Lane Kolbly