CHOP-763: Extend LLEventAPI "LLViewerControl" API; add queries.

This is a significant refactoring of planned (but as yet unimplemented) work,
though in fact it's almost completely compatible with the only implemented
operation. The set() operation now requires op="set", where before that was
inferred because set() was the only possibility.
Whereas before LLViewerControlListener dispatched to different bound methods
on the "group" key, with four known "group" string values, it now dispatches
on the "op" key, supporting "set", "toggle", "get", "groups", "vars" -- the
last two exposing query functionality. LLControlGroup is actually derived from
LLInstanceTracker, keyed on string names, so we can look up instances using
LLControlGroup::getInstance(const std::string&), or enumerate all such names.
LLControlGroup similarly permits iterating over all defined LLControlVariables.
The static LLViewerControlListener instance has been wrapped in an unnamed
namespace and removed from llviewercontrollistener.h. The availability of the
API depends on LLEventPumps::obtain(), rather than normal C++ visibility.