Moving internal constants to Liquid.h

Signed-off-by: Miguel M. Almeida <miguelcma@CMA-Retina.local>
New event _restoreSession

Event _restoreSession is now automatically tracked by Liquid.
This helps Liquid backend to improve session length accuracy.

Signed-off-by: Miguel M. Almeida <miguelcma@CMA-Retina.local>