This is in line with how the other APIs are named (for example
ObjectDataXD have instance type and instance). This would be very hard
to change later without breaking backwards compatibility, so I'm doing
it now, until the animation APIs get widely used.
Note that I don't mean all possible underlying types, just all possible
vector classes. This is needed for splines etc. which may be templated
and have result type a generic vector, for example.
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.