Public access to JEvent::Set

[DraTeots]

Related to #154

JEvent::Set is publicly open while should only be used for EventSources. Might be very confusing for users that

// in one place
event->Set(...);   // Allowed and OK

// in another place
event->Set(...);   // Allowed but is NOT OK

Here I intentionally use "one place" and "another place" instead of EventSource and Factory because for new users it is "one place" to look and copy code and "another place" to look and copy code.

[nathanwbrei]

Moving this discussion to #254