Adding an examples that implements a twin stick controller, with MKB/gamepad input

[AtheMathmo]

This is a fairly complete example that uses several of the concepts present in other examples.

The main new thing is the InputMode state, which detects MKB/gamepad input and switches an app state accordingly.

I also use an ActionState Resource here, as in #413 . I can change this to a component-based approach if preferred, but I think it gives quite a nice way to handle ActionData mutation without a driver.

I also wanted to add in some visuals here (a plane, a cube, etc.). But the crate doesn't include any of those dependencies from bevy. I didn't want to add these as dev dependencies, but happy to implement this too if you think it is worthwhile.

I'm still figuring things out, so happy to make any changes as requested.