Carriage inventory tests

This is all still placeholder/test art (especially the items) - but today I got a basic inventory interaction working, allowing you to transfer items from a station into your train. The train is also now properly tracking how much capacity each type of carriage has.

The image shows a station offering milk and coal, and the train having 100% filled filled both of its milk carriages.

Right now, clicking an item type transfers the maximum amount into the train - next I need to add a proper click/drag of items in increments.