NPCs, shops, inventories

I've now broken out inventories and inventory interactions so you can view your own inventory separate from any other UI.

There is now a 'purchase' mode that treats another inventory as a store - so I added the first NPC in the game (although they are a totally static shopkeeper) from which you can now buy items:

Looking past the hideous prototype art, this is another big step toward functioning gameplay. The pictured shop, while currently selling milk as a test, is intended to be a 'wagon workshop' that will build new wagons/carriages for your train. That unlocks more and different capacity to carry more valuable cargo. This is intended to be (fingers crossed) a crucial part of drawing the player into the game.

The next step toward this is actually properly saving the state and position of the train, something I have avoided until now. Doing this will unlock the ability to add wagons onto the train with some degree of permanence.

Bonus: You may have noticed there is now time of day too. I need to spend a few hours at some point to add day/night lighting.