* Added an inventory system for actors as a replacement to the arbitrary
key/value data store. Colored keys now add themselves to the player's
inventory, and colored doors check the inventory.
* Inventory is a map[string]int between doodad filenames
(red-key.doodad) and quantity (0 for key items/unlimited qty).
* API methods to add and remove inventory.
* Items HUD appears in Play Mode in lower-left corner showing doodad
sprites of all the items in the Player's inventory.
