Bug in the event objects handled to keyboard and mouse events

TypeScript has different Event type objects depending on the type of event being handled. This means that the signature for the events in the Stage and Scene class are wrong; they use Event and not KeyboardEvent and MouseEvent (respectively).

This is not super critical because you can just change the signature when you override and everything works as expected, but it does get in the way of the IDE auto-generating overrides and such, and it makes the documentation wrong.