This reduces the templated code a bit, as I moved the index assertion to
the *.cpp file. Also the function now returns a reference to avoid
needless copies -- it's a view, but still quite a heavy view.
Moreover, this will prevent from passing e.g. integer-based keys to
Trade::AnimationData. And this also now allows me to add duration() to
Trade::AnimationData. I also moved all accessors that don't need a
concrete value type to this base class.