Simplify token creation

[lilleyse]

The previous approach for adding new tokens was overly complex, requiring changes in 5 different places within Tokens.h and Tokens.cpp. Now it's as simple as adding a new token to USD_TOKENS and letting the macros do the rest of the work.

USD tokens are now created in a more standard way with TF_DECLARE_PUBLIC_TOKENS and TF_DEFINE_PUBLIC_TOKENS and are accessed as pxr::UsdTokens->tokenName.

Fabric tokens are created with FABRIC_DECLARE_TOKENS and FABRIC_DEFINE_TOKENS which are inspired by the pxr macros and are accessed as FabricTokens::tokenName like before.

[weegeekps]

Lots cleaner.