return a reference to an actual object in currentContext()

So far it had returned a reference to a nullptr to avoid anybody somehow doing anything with it, but aparently by now gcc checks that at run-time during debug builds, so this will now provide an actual object and also provides a dummy class definition. I added the later as in the past I had seen people define their own version of EventContext which would create problems when used with a concrete object, and this at least gives the compiler a chance to catch that.

This should fix the error in debug mode reported in ATLASG-1634.