ER-710:  Viewer effects.   Added 'lookAt' command for LLAgent event host API.  Also changed a bunch of parameters named 'event' to 'event_host' since VS highlights them as keywords.  Reviewed by Kelly.

Added optional 'position' property to 'requestSit' LLEventHost function, re-factored code into new findObjectClosestTo() function.  Reviewed by Kelly

ER-650:  touch events.  Added 'requestTouch' API for LLAgent operations via llEventHost.   Re-factored some message sending code for grabs.  Reviewed by Kelly

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.