Introduce LLEventDispatcher::begin()/end() to iterate over (name, desc) pairs

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.