Add functions sprite:bring_to_front() and sprite:bring_to_back().

If the given sprite belongs to some entity (say, to a custom entity, to the hero, or to any other entity), then the methods above should make the sprite to be drawn before/after all the other sprites of the entity.

In case the sprite does not belong to an entity (I don't know if this is possible now, but I suppose it will be someday), then these functions should work exactly as the entity functions entity:bring_to_front() and entity:bring_to_back().