Implement OpenXR hand tracking

Add an OpenXRHandTracking class, which encloses the OpenXR calls needed
to use hand tracking (check availability, create/destory trackers, get
joints matrices), and also manage the 2 hands 3D models.

Currently, it only implements 'simple' hand tracking.
Additional extensions (to support hand tracking while holding the
controllers) is still to be finalized (related code is commented).