Expose per-feaure events through ISensor

This makes it possible to attach ground term handlers directly to the
event they are interested in.
Do not create objects upon transitions

Instead of pairing feature and event in a SensorEventArgs and passing
it to completion/gesture events, they can just be passed directly.

This removes the need for the SensorEventArgs type.
Make features more generic

Features can now be of any type, because GestIT uses them in an opaque
way (the only operation performed by GestIT on features is to query
the sensor for a feature-specific event).