added walk_speed property
removed unnecessary actions for START and STOP states in the sub state machine
added the walk and nap states for the patrol sub state machine
modifies the 'properties' member to use an instance of the  'EnemyProperties' class
added the 'EnemyProperties' class
passes the elapsed time as argument to the STEP_GAME action
added the 'ActionKeys' class corresponding to the level actions
modified the member 'player_properties' to the more generic name 'properties'
passes the elapsed time since the previos clock tick to the 'step_game' method