Reduce Event.h and EventQueue.h using C++11

Variadic templates can reduce Event.h from 4,100 lines to 300, and
EventQueue.h from 3,400 to 1,000, so 6,000 lines saved total.

End result isn't totally variadic, as we still need specialisations
for storing 0-5 values in contexts, but that specialisation is now
in exactly one place.

Only change other from switching to variadic templates is using
delegating constructors instead of the `new (this)` trick. That trick is
still used in the assignment operator.

Minor documentation correction. It's possible that the separate
simplified variadic Doxygen version not be needed now, but I've left it.

Author: kjbracey