Needed: NPCs

[jonoxia]

At their most basic, an NPC is just:

• A sprite that is drawn on the map
• It can be stationary or programmed to wander around
• Its walking animation should not interfere with the player's walking animation
• Detect collisions - prevent player from walking through NPCs.
• Set an onTalk callback for the NPC. Trigger it when the player faces the NPC and presses the talk button. This could be as simple as showing a single message, or as complex as opening a unique menu system instance with options that modify the player inventory (aka a shop).

[jonoxia]

Remaining NPC issues:

• NPC can currently walk through anything. We could fix this on an individual basis by setting the canCross method of every NPC, but that's a lot of redundancy. Would be better if we could set a default canCross method that is applied to all PlayerCharacters and NPCs unless overridden.
• NPC can walk through PC (though not vice-versa). The map.canMove method needs to have access to the party's locations and not allow NPCs to enter squares where a PC is standing -- although PCs should always be able to walk through each other!
• NPC class duplicates a lot of animation code from PlayerCharacter class. As with canCross, we really want a way to set a default walkAnimation used for all PCs and NPCs unless overidden. And then we can move some of the walk animation generator to MapSpriteMixin.

[jonoxia]

Code duplication solved in the last refactoring. Starting new bugs for the movement issues.