Inventory system added

Added a basic inventory system that uses the item's name as a key.

Players can obtain items, check how much of a type of item he possesses,
and lose items.
Found a sprite sheet with most of the required animations.
Finished the player character sprite sheet

Adjusted the existing 8 directions of flying animation, and drew the 8
directions of walking and idling.  Implemented the sprites to fully
animate the player character.  Some fine-tuning is needed, but this is a
major milestone nevertheless.
Merge animation branch
Add quest system prototype