Quests: Allow for tasks to give multiple rewards upon completion

[elizabethli31]

Quest rewards can currently only give one item, but it would likely be better if they can give the player multiple items. A good task to start with would be replacing the current reward item implementation with a linked list of item pointers instead of just a single pointer. This would involve changing the item struct (and the management functions, including new, init, and free), the accept reward function, and the reward WDL parsing function.