Apparently the outer class's LL_COMMON_API marker affects all outer class
members, but not nested classes. Making it explicit fixes Windows link errors.

This is a generally-useful idiom, extending the sendReply() convenience
function -- it shouldn't remain buried in a single .cpp file.

for all registered operations. (untested)
Introduce LLEventDispatcher::getMetadata(name) query so you can discover, for
a given named operation, its query string and required parameters. (untested)
Introduce LLEventDispatcher::add() convenience methods allowing you to omit
description strings. Fix LLLoginInstance (which uses a non-LLEventAPI
LLEventDispatcher) back to description-less add() calls.
However, filter LLEventDispatcher::add() methods inherited by LLEventAPI so
that an LLEventAPI subclass *must* provide a description string.

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.