Gameplay + Physic: beginning
 * first pieces of gameplay
 * physic now handle time management:  a brick makes
PhysicSolver.TURN_DURATION_MS ms to fall
 * Gameplay workflow improvement: PhysicSolver.resolvePhysic() is no
longer interrupted by GameBoard when it notifies its listeners. 
 * User events are not tied to the PhysicSolver.TURN_DURATION_MS and can
happen any time.
 * Introduce Score: a class holding all the information and computations
relative to the GameBoard score.
 * GameBoard: new methods: isBoardFull() and int getScore()