Previously, LLEventAPI intentionally hid all but one of the many add()
overloads supported by its LLEventDispatcher base class. The reason was that
certain of the add() methods take an optional fourth parameter that's an
LLSD::Map describing the expected parameter structure, while others take a
fourth templated parameter that's an instance getter callable. This led to
ambiguity, especially when passed an LLSDMap instance that's convertible to
LLSD but isn't literally LLSD. At the time, it was simpler to constrain the
add() methods inherited from LLEventDispatcher.

But by adding new std::enable_if constraints to certain LLEventDispatcher
add() methods, we've resolved the ambiguities, so LLEventAPI subclasses can
now use any add() overload (as claimed on the relevant Confluence page).

LLEventDispatcher comments have always loftily claimed that an instance getter
callable may return either a pointer or a reference, doesn't matter. But it
does when trying to pass the getter's result to boost::fusion::push_back(): a
reference must be wrapped with std::ref() while a pointer cannot be.
std::ref(pointer) produces errors. Introduce LLEventDispatcher::invoker::
bindable() overloads to Do The Right Thing whether passed a pointer or a
reference.

(cherry picked from commit 743f487c2e123171c9fc6d5b84d768f1d856d569)