extensions: import the exthelper class from evolve 980565468003 (API) This should help make extensions that wrap a lot of stuff more comprehendible. It was copied unmodified, except: - fix up the imports - rename final_xxxsetup() -> finalxxxsetup() to appease checkcode - avoid a [] default arg to wrapcommand() .. api:: Add `exthelper` class to simplify extension writing by allowing functions, commands, and configitems to be registered via annotations. The previous APIs are still available for use.