Enabling chained inherits from State<> - so users can make/use their own BaseState<>

[vincentnl]

Users may want to inherit in multiple steps from State<>. These grand-children are currently not added to DI, as their generic basetype is not State<>.

I'm a little fuzzy how you are testing/your test setup (Xunit experience here), or I would have added tests. Let me know.

[StevenTCramer]

This seems reasonable to me at first glance. @mikeyoshino please take a look and let me know your thoughts.