Add LLEventAPI class, formalizing the mechanism by which we wrap a C++ API

with an event API. In addition to the LLEventPump name on which to listen,
LLEventAPI accepts a documentation string for event API introspection.
Give every LLEventDispatcher::add() overload a new documentation string
parameter for event API introspection.
Convert every existing event API to new conventions, introducing suitable
documentation strings for the API and each of its operations.