Define named values for mtsIdx and use them

source/Lib/CommonLib/ContextModelling.cpp

source/Lib/CommonLib/DepQuant.cpp

source/Lib/CommonLib/Quant.cpp

source/Lib/CommonLib/QuantRDOQ.cpp

source/Lib/CommonLib/TrQuant.cpp

source/Lib/CommonLib/TypeDef.h

source/Lib/CommonLib/Unit.cpp

source/Lib/CommonLib/UnitTools.cpp

source/Lib/DecoderLib/CABACReader.cpp

source/Lib/EncoderLib/CABACWriter.cpp

source/Lib/EncoderLib/EncCu.cpp

source/Lib/EncoderLib/InterSearch.cpp

source/Lib/EncoderLib/IntraSearch.cpp