Make the delay before entering Pushing state customizable

• Added a int push_delay member to the Hero class, and its getter/setter. This value is used as the delay between when the hero starts pushing onto a wall (i.e. the hero faces a wall and the player still pressed the direction towards this wall) and when the Pushing state begins, instead of a hardcoded value of 800 ms (the default value of push_delay is still 800).
• Added two methods to the hero meta (hero:get_push_delay() and hero:set_push_delay(delay)), invoking the getter/setter.